feat: add per-layer LED range and collapsible layers to composite source

Composite layers now support optional start/end LED range (toggleable)
and reverse flag, making composite a superset of mapped source. Layers
are collapsible with animated expand/collapse and consistent 0.85rem
font sizing. Delete button restyled as ghost icon button.

Also includes minor dashboard CSS overflow fixes.

TODO-css-improvements.md

@@ -11,6 +11,15 @@
 
 ---
 
+## Donation / Open-Source Banner
+
+- [ ] Add a persistent but dismissible banner or notification in the dashboard UI informing users that the project is open-source and under active development, and that donations are highly appreciated
+- [ ] Include a link to the donation page (GitHub Sponsors, Ko-fi, or similar — decide on platform)
+- [ ] Remember dismissal in localStorage so it doesn't reappear every session
+- [ ] Add i18n keys for the banner text (`en.json`, `ru.json`, `zh.json`)
+
+---
+
 # Color Strip Source Improvements
 
 ## New Source Types
@@ -35,7 +44,6 @@ Need to research HAOS communication options first (WebSocket API, REST API, MQTT
 
 - [x] Add effects: rain, comet, bouncing ball, fireworks, sparkle rain, lava lamp, wave interference
 - [x] Custom palette support: user-defined [[pos,R,G,B],...] stops via JSON textarea
-- [ ] Custom palette editor: replace raw JSON textarea with a proper visual editor (reuse gradient editor pattern with color pickers + position sliders)
 
 ### `gradient`
 
@@ -63,7 +71,7 @@ Need to research HAOS communication options first (WebSocket API, REST API, MQTT
 
 - [ ] Allow nested composites (with cycle detection)
 - [ ] More blend modes: overlay, soft light, hard light, difference, exclusion
-- [ ] Per-layer LED range masks
+- [x] Per-layer LED range masks (optional start/end/reverse on each composite layer)
 
 ### `notification`