Feature: Selectable player themes (ambient + immersive layouts)

[pizza-init]

Summary

Add a theme system to the player UI — multiple selectable visual themes, switchable from Settings → Display, persisted to NVS, applied on boot. The current look stays as the untouched default; new themes are added alongside it.

Design mockups

Theme A — "Ambient" (theme-02.png)

• Refined version of the current layout.
• Background tinted from the album art's ambient/dominant colour (deep purple-pink for pink art, etc.) instead of the static dark grey.
• Left: rounded, tinted album art. Right: pink artist, large white title, album · year, progress bar, transport row, volume slider.
• Synced lyrics bottom-left with the current line highlighted.

Theme B — "Immersive" (theme-03.png)

• A different, lyrics-forward layout (not just recolouring).
• Solid saturated ambient colour fills the whole screen (derived from album art).
• Compact header: small art thumbnail + artist + "Living room · playing"; settings/queue top-right.
• Oversized title typography spanning the screen.
• Large translucent background lyric text + highlighted current synced lyric.
• Minimal bottom bar: progress + transport (pink play/pause).

Requirements

• Data-driven / extensible — adding a new theme should be a minimal change (one registry entry), not a rewrite. The settings list and the render logic iterate the registry. (An earlier quick attempt was reverted specifically for not being extensible — this is the key requirement.)
• Selectable in Settings (Display), choice saved to NVS, applied on boot (clamp invalid/stale index).
• Ambient colour derived from album art — machinery already exists (dominant_color, setBackgroundColor(), g_ambient_bright, blur bg).
• Keep the current theme as the default, unchanged.
• Themes can differ in background treatment AND layout (Theme B is a distinct layout, so themes may need to select different screen builders, not just a colour palette).

Open questions

• Theme B's big-typography + background-lyrics layout — feasibility/perf on the P4 (PPA is currently disabled; all rendering is software).
• Naming for the themes in the settings list.

References

• Mockup sources: theme/theme-02.png, theme/theme-03.png (committed on branch theme-mockups).

[johnhenrick3-cpu]

Looks excellent