Merge Luma launcher fork into monorepo

Move trampoline files (boot-hook, watcher, overlay) into trampoline/
subdirectory. Add upstream Luma v2.3.1 source at repo root with our
additions:

- Page::Ha (4th nav tab) with placeholder empty state
- HA tab config schema and default entity mapping (ha-home/)
- Nav keys renamed: "home" → "start", new "ha" tab between Apps and Settings
- All state.rs/view.rs/main.rs/tests.rs updated for 4-tab navigation

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

assets/ui-icons/spotify.svg

@@ -0,0 +1 @@
+<svg viewBox="0 0 48 48" xmlns="http://www.w3.org/2000/svg" width="128" height="128" color="#fff"><circle cx="24" cy="24" r="24" fill="white"/><g fill="none" stroke="#15271c" stroke-linecap="round"><path d="M11 18q15-6 27 2" stroke-width="4"/><path d="M13 25q13-5 23 2" stroke-width="3.5"/><path d="M15 32q10-4 18 1" stroke-width="3"/></g></svg>