theme: preserve CSS alpha in hex color parsing on both platforms

[0xSero]

Follow-up to #312: theme colors written as CSS hex with an alpha channel lost their translucency.

Problem: CSS/VS Code hex is alpha-last (#RGB, #RGBA, #RRGGBB, #RRGGBBAA). Android's Color(Long) is ARGB (alpha first), and the old Android parser fed the hex digits straight into Color(Long) — so #4585AA80 rendered as 0x804585AA-misread opaque color instead of translucent #4585AA at 50% alpha. iOS's Scanner-based helper silently mis-parsed or dropped the alpha byte for the same shapes.

Changes:

• Android colorFromHex: expand 3/4-digit shorthands, parse 6/8-digit forms, and move the alpha byte to the ARGB front for 8-digit input; invalid input falls back cleanly (no Android framework Color.parseColor dependency in the hot path).
• iOS: shared LitterHexRGBA helper (top-level in LitterPalette.swift because the Live Activity extension compiles that file but not Extensions.swift) parses alpha-last hex; Color(hex:), UIColor(hex:), ThemeDefinition.hexToRGB, and the watch's WatchThemeStore (which compiles neither shared file) all preserve alpha now.
• QA matrix row + tests on both platforms.

Verification:

• Android: ThemeColorTest — #4585AA80 → 0x804585AA, #A8AA shorthand → 0xAAAA88AA, opaque token still matches the generated Material role, invalid input falls back without the framework parser.
• iOS: HexColorTests on the simulator — 6 tests, 0 failures (alpha preserved, shorthand expansion, uppercase/padded input, invalid → nil, UIColor alpha, LitterPalette.Pair.colorFromHex).