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/components/SettingsModal.tsx

@@ -1,5 +1,6 @@
 import { useEffect, useMemo, useRef, useState } from "react";
 import { CleanupOrphanedThumbnailsResult, DatabaseInfo, OrphanedThumbnailsInfo, TaggerAcceleration, TaggingQueueScope, VacuumResult, useGalleryStore } from "../store";
+import { FfmpegStatusRow } from "./onboarding/StepWelcome";
 
 type SettingsSection = "workspace" | "general";
 
@@ -184,6 +185,7 @@ export function SettingsModal() {
   const updateError = useGalleryStore((state) => state.updateError);
   const checkForUpdates = useGalleryStore((state) => state.checkForUpdates);
   const installUpdate = useGalleryStore((state) => state.installUpdate);
+  const openOnboarding = useGalleryStore((state) => state.openOnboarding);
 
   useEffect(() => {
     if (!settingsOpen) return;
@@ -625,6 +627,24 @@ export function SettingsModal() {
                   </SettingsItem>
                 </SettingsGroup>
 
+                <SettingsGroup title="Setup">
+                  <FfmpegStatusRow />
+                  <SettingsItem
+                    label="Welcome tour"
+                    description="Replay the guided first-run tour — library setup, the background pipeline, search modes, and AI features."
+                  >
+                    <button
+                      className="rounded-md border border-white/10 bg-white/[0.055] px-3 py-1.5 text-xs text-gray-300 transition-colors hover:bg-white/10 hover:text-white"
+                      onClick={() => {
+                        setSettingsOpen(false);
+                        openOnboarding();
+                      }}
+                    >
+                      Show welcome tour
+                    </button>
+                  </SettingsItem>
+                </SettingsGroup>
+
                 <SettingsGroup title="Storage & notifications">
                   <SettingsItem
                     label="App data folder"