0.8.1: dirty-state + Cmd/Ctrl+S, fullscreen plays to OUT, UX fixes

- Unsaved-changes indicator: a small accent dot appears on the save
  button whenever the scene has changed since the last save/load.
  Save/load/new all mark the project clean on next tick.
- Cmd/Ctrl+S saves the project (reuses the same flow as the button;
  prompts for a name on first save). Tooltip now mentions the
  shortcut.
- Fullscreen preview plays to the OUT marker (or scene.duration when
  no OUT is set) instead of looping at the last keyframe. Dropped
  the runtime's effectiveEnd helper; RUNTIME_VERSION 13 → 14.
- "Still frame" keyframes no longer disable easing. A held kf's
  easing still drives the segment INTO it from the previous kf, so
  you can ease into a pause. Only the first kf disables easing
  (nothing precedes it). Dropped `timeline.holdDisabledHint`.
- Settings dialog: hint text 11px → 13px (and dropped italic) so
  the inline help is readable; section headings 10px → 13px.
- Canvas stage shadow: `shadowCfg` always emits explicit values plus
  a `shadowEnabled` flag instead of returning `{}`. Konva's default
  shadowOpacity is 1, so bailing with `{}` could leave stale shadow
  attrs on the shape. Preview and editor now gate on the same
  condition deterministically.
- Sidebar empty-state copy: "No clips" → "No clips yet" to match
  "No buttons yet".

Regression tests: runtime rangeOut fallback is scene.duration (not
last kf); sidebar empty-states both contain "yet"; the
holdDisabledHint key is gone; saveProject tooltips mention the
shortcut.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/i18n/messages.ts

@@ -7,7 +7,8 @@ export const en = {
     play: 'Play (Space)',
     stop: 'Stop',
     recentProjects: 'Recent projects',
-    saveProject: 'Save project (browser storage)',
+    saveProject: 'Save project (Ctrl/Cmd+S)',
+    saveProjectDirty: 'Save project (unsaved changes, Ctrl/Cmd+S)',
     documentSettings: 'Document settings',
     export: 'Export',
     mainTimeline: 'Main timeline',
@@ -66,7 +67,7 @@ export const en = {
     clipLayerCount: '{n} layers',
     renameTooltip: 'Rename',
     deleteTooltip: 'Delete',
-    noClips: 'No clips',
+    noClips: 'No clips yet',
     shape: {
       rect: 'Rect',
       circle: 'Circle',
@@ -243,7 +244,6 @@ export const en = {
     easingEaseOut: 'Ease Out',
     easingEaseInOut: 'Ease In-Out',
     firstKfHint: 'First keyframe — easing has no effect',
-    holdDisabledHint: 'Still frame — easing disabled',
     stillFrame: 'Still frame',
     duplicate: 'Duplicate',
     delete: 'Delete',