Add noise gate, palette quantization filters and drag-and-drop filter ordering

- Add noise gate filter: suppresses per-pixel color flicker below threshold
  using stateful frame comparison with pre-allocated int16 buffers
- Add palette quantization filter: maps pixels to nearest color in preset
  or custom hex palette, using chunked processing for memory efficiency
- Add "string" option type to filter schema system (base, API, frontend)
- Replace up/down buttons with pointer-event drag-and-drop in PP template
  filter list, with clone/placeholder feedback and modal auto-scroll
- Add frame_interpolation locale keys (was missing from all 3 locales)
- Update TODO.md: mark completed processing pipeline items

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

TODO.md

@@ -4,11 +4,11 @@ Priority: `P1` quick win · `P2` moderate · `P3` large effort
 
 ## Processing Pipeline
 
-- [ ] `P1` **Noise gate** — Suppress small color changes below threshold, preventing shimmer on static content
-- [ ] `P1` **Color temperature filter** — Warm/cool shift separate from hue shift (circadian/mood)
+- [x] `P1` **Noise gate** — Suppress small color changes below threshold, preventing shimmer on static content
+- [x] `P1` **Color temperature filter** — Already covered by existing Color Correction filter (2000-10000K)
 - [ ] `P1` **Zone grouping** — Merge adjacent LEDs into logical groups sharing one averaged color
-- [ ] `P2` **Palette quantization** — Force output to match a user-defined palette
-- [ ] `P2` **Drag-and-drop filter ordering** — Reorder postprocessing filter chains visually
+- [x] `P2` **Palette quantization** — Force output to match a user-defined palette (preset or custom hex)
+- [x] `P2` **Drag-and-drop filter ordering** — Reorder postprocessing filter chains visually
 - [ ] `P3` **Transition effects** — Crossfade, wipe, or dissolve between sources/profiles instead of instant cut
 
 ## Output Targets