feat(onboarding): guided first-run tour with background FFmpeg provisioning

Phase 5 of the 0.1.0 release prep:
- FFmpeg no longer blocks startup (or crashes the app offline): provisioning
  runs in a background thread emitting throttled ffmpeg-progress events,
  with installed/downloading/failed state queryable via get_ffmpeg_status
  and a retry command
- video jobs are invisible to claiming and tier-priority checks until FFmpeg
  is ready, so they wait as pending without failing — and without starving
  image embeddings/tagging (include_videos gating in db.rs)
- 7-step show-don't-tell onboarding wizard: welcome + live FFmpeg progress,
  real first-folder picker, faked animating pipeline bar, placeholder
  gallery tiles, cycling search-syntax demo, views overview, and an AI
  features step with a real opt-in tagger download
- skippable at every point (Escape included), persisted via
  settings/onboarding_completed.txt, re-runnable from Settings > General,
  which also gains an FFmpeg status/retry row

src-tauri/src/media.rs

@@ -2,9 +2,136 @@ use anyhow::{anyhow, Result};
 use ffmpeg_sidecar::download::{auto_download_with_progress, FfmpegDownloadProgressEvent};
 use ffmpeg_sidecar::ffprobe::ffprobe_path;
 use ffmpeg_sidecar::paths::ffmpeg_path;
-use serde::Deserialize;
+use serde::{Deserialize, Serialize};
 use std::path::{Path, PathBuf};
 use std::process::Command;
+use std::sync::atomic::{AtomicBool, Ordering};
+use std::time::{Duration, Instant};
+use tauri::{AppHandle, Emitter};
+
+static FFMPEG_READY: AtomicBool = AtomicBool::new(false);
+static FFMPEG_DOWNLOADING: AtomicBool = AtomicBool::new(false);
+// Set when a provision attempt fails, so the frontend can distinguish
+// "failed before the event listener attached" from "about to start".
+static FFMPEG_FAILED: AtomicBool = AtomicBool::new(false);
+
+/// True once both ffmpeg and ffprobe binaries are present on disk. Workers
+/// gate video jobs on this so a missing/in-flight download never fails jobs.
+pub fn ffmpeg_ready() -> bool {
+    FFMPEG_READY.load(Ordering::Relaxed)
+}
+
+pub fn ffmpeg_downloading() -> bool {
+    FFMPEG_DOWNLOADING.load(Ordering::Relaxed)
+}
+
+pub fn ffmpeg_failed() -> bool {
+    FFMPEG_FAILED.load(Ordering::Relaxed)
+}
+
+#[derive(Debug, Clone, Serialize)]
+pub struct FfmpegProgressPayload {
+    pub phase: String,
+    pub downloaded_bytes: Option<u64>,
+    pub total_bytes: Option<u64>,
+    pub error: Option<String>,
+}
+
+impl FfmpegProgressPayload {
+    fn phase(phase: &str) -> Self {
+        Self {
+            phase: phase.to_string(),
+            downloaded_bytes: None,
+            total_bytes: None,
+            error: None,
+        }
+    }
+}
+
+const FFMPEG_PROGRESS_EVENT: &str = "ffmpeg-progress";
+
+/// Provision FFmpeg in the background, streaming progress to the frontend as
+/// `ffmpeg-progress` events. Idempotent: re-invoking while a download is in
+/// flight is a no-op, and re-invoking after success only re-emits `done`.
+pub fn spawn_ffmpeg_provision(app: AppHandle) {
+    // Fast path: binaries already on disk (previous run, or a manual install).
+    if ffmpeg_path().exists() && ffprobe_path().exists() {
+        FFMPEG_READY.store(true, Ordering::Relaxed);
+        let _ = app.emit(FFMPEG_PROGRESS_EVENT, FfmpegProgressPayload::phase("done"));
+        return;
+    }
+
+    if FFMPEG_DOWNLOADING
+        .compare_exchange(false, true, Ordering::SeqCst, Ordering::SeqCst)
+        .is_err()
+    {
+        return; // a download is already running
+    }
+    FFMPEG_FAILED.store(false, Ordering::SeqCst);
+
+    std::thread::spawn(move || {
+        // The Downloading callback fires very frequently; throttle emissions.
+        // Cell because the download callback is Fn, not FnMut.
+        let last_emit = std::cell::Cell::new(Instant::now() - Duration::from_secs(1));
+        let result = auto_download_with_progress(|event| match event {
+            FfmpegDownloadProgressEvent::Starting => {
+                log::info!("Downloading bundled FFmpeg...");
+                let _ = app.emit(
+                    FFMPEG_PROGRESS_EVENT,
+                    FfmpegProgressPayload::phase("starting"),
+                );
+            }
+            FfmpegDownloadProgressEvent::Downloading {
+                total_bytes,
+                downloaded_bytes,
+            } => {
+                if last_emit.get().elapsed() >= Duration::from_millis(250) {
+                    last_emit.set(Instant::now());
+                    let _ = app.emit(
+                        FFMPEG_PROGRESS_EVENT,
+                        FfmpegProgressPayload {
+                            phase: "downloading".to_string(),
+                            downloaded_bytes: Some(downloaded_bytes),
+                            total_bytes: Some(total_bytes),
+                            error: None,
+                        },
+                    );
+                }
+            }
+            FfmpegDownloadProgressEvent::UnpackingArchive => {
+                log::info!("Unpacking bundled FFmpeg...");
+                let _ = app.emit(
+                    FFMPEG_PROGRESS_EVENT,
+                    FfmpegProgressPayload::phase("unpacking"),
+                );
+            }
+            FfmpegDownloadProgressEvent::Done => {
+                log::info!("Bundled FFmpeg ready.");
+            }
+        });
+
+        FFMPEG_DOWNLOADING.store(false, Ordering::SeqCst);
+        match result {
+            Ok(()) => {
+                FFMPEG_READY.store(true, Ordering::Relaxed);
+                let _ = app.emit(FFMPEG_PROGRESS_EVENT, FfmpegProgressPayload::phase("done"));
+            }
+            Err(error) => {
+                FFMPEG_FAILED.store(true, Ordering::SeqCst);
+                log::error!("FFmpeg provisioning failed: {error}");
+                let _ = app.emit(
+                    FFMPEG_PROGRESS_EVENT,
+                    FfmpegProgressPayload {
+                        phase: "error".to_string(),
+                        downloaded_bytes: None,
+                        total_bytes: None,
+                        error: Some(error.to_string()),
+                    },
+                );
+            }
+        }
+    });
+}
 
 // On Windows, GUI apps spawn subprocesses with a visible console window by default.
 // CREATE_NO_WINDOW suppresses that for every ffmpeg/ffprobe invocation.
@@ -36,30 +163,6 @@ impl MediaTools {
         }
     }
 
-    pub fn ensure_installed() -> Result<()> {
-        // Skip download entirely if both binaries are already present.
-        if ffmpeg_path().exists() && ffprobe_path().exists() {
-            return Ok(());
-        }
-        auto_download_with_progress(|event| match event {
-            FfmpegDownloadProgressEvent::Starting => {
-                log::info!("Downloading bundled FFmpeg...");
-            }
-            FfmpegDownloadProgressEvent::Downloading {
-                total_bytes,
-                downloaded_bytes,
-            } => {
-                log::info!("Downloading bundled FFmpeg: {downloaded_bytes}/{total_bytes} bytes");
-            }
-            FfmpegDownloadProgressEvent::UnpackingArchive => {
-                log::info!("Unpacking bundled FFmpeg...");
-            }
-            FfmpegDownloadProgressEvent::Done => {
-                log::info!("Bundled FFmpeg ready.");
-            }
-        })
-    }
-
     pub fn ffmpeg_command(&self) -> Command {
         let mut cmd = Command::new(&self.ffmpeg_path);
         #[cfg(target_os = "windows")]