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/components/DocumentSettings.vue

@@ -454,11 +454,11 @@ section {
 
 section h3 {
   margin: 0;
-  font-size: 10px;
+  font-size: 13px;
   text-transform: uppercase;
-  color: var(--text-dim);
+  color: var(--text-dim-strong);
   letter-spacing: 0.08em;
-  font-weight: 500;
+  font-weight: 600;
 }
 
 .row {
@@ -558,9 +558,9 @@ input[type='number'] {
 }
 
 .hint {
-  font-size: 11px;
-  color: var(--text-dim);
-  font-style: italic;
+  font-size: 13px;
+  line-height: 1.45;
+  color: var(--text-dim-strong);
   margin: 0;
 }