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/Timeline.vue

@@ -2825,13 +2825,13 @@ onBeforeUnmount(() => {
           v-for="opt in EASING_OPTIONS"
           :key="opt.value"
           class="kf-menu-item"
-          :class="{ active: menu.currentEasing === opt.value && !menu.isHeld }"
-          :disabled="menu.isFirst || menu.isHeld"
+          :class="{ active: menu.currentEasing === opt.value }"
+          :disabled="menu.isFirst"
           @click="applyEasing(opt.value)"
         >
           <span class="kf-menu-icon">
             <PhCheck
-              v-if="menu.currentEasing === opt.value && !menu.isHeld"
+              v-if="menu.currentEasing === opt.value"
               :size="12"
               weight="bold"
             />
@@ -2839,7 +2839,6 @@ onBeforeUnmount(() => {
           {{ opt.label }}
         </button>
         <div v-if="menu.isFirst" class="kf-menu-note">{{ t('timeline.firstKfHint') }}</div>
-        <div v-else-if="menu.isHeld" class="kf-menu-note">{{ t('timeline.holdDisabledHint') }}</div>
       </div>
       <div class="kf-menu-divider"></div>
       <button