feat: color palettes

[frysee]

Let users pick from predefined, file-based color themes (shippable + community-shareable)
while keeping full per-color custom editing. A theme = the 7 UI colors (color1–7).

File format

Reuse the existing key=value / colorN=0xRRGGBBAA syntax (parser already exists —
parseHexColor, accepts RGB and RGBA). One theme = one .txt file.

version=1
name=Default
color1=0xffffffff
color2=0x9b2257ff
color3=0x1e2329ff
color4=0xffffffff
color5=0x000000ff
color6=0xffffffff
color7=0x000000ff

• version= = theme-format version (start at 1). Lets us extend the schema later
  (e.g. add font/radius). Missing version ⇒ treat as 1. A theme whose version
  exceeds the max we support is skipped entirely (not listed, not applied).
• name= = display label (fallback: filename minus extension, underscores→spaces).
• Scope is colors only for now; Unknown keys ignored (forward-compat).

Storage layout — dual folders

• Built-in themes: ship in skeleton/SYSTEM/res/themes/*.txt (→ RES_PATH/themes,
  controlled, survives updates, not meant to be user-edited).
• Community/user themes: drop-in folder SDCARD_PATH/Palettes/*.txt (top-level, visible,
  survives settings reset). Add skeleton/BASE/Palettes/ with a short README explaining
  the file format so users can author/share.
• Enumeration scans both; built-ins listed first. On name collision, keep both but tag
  source (built-in vs user).