Stillsuit: Omarchy-style global theming to replace Stylix

[Kabilan108]

Sub-task of #17. Lower priority; not scheduled.

Goal

Replace Stylix as the theming source of truth with a Stillsuit-owned rich theme format, in the style of Omarchy's theme directories. Stillsuit already compiles its own three-level theme (palette → semantic roles → component assignments) via packages/stillsuit-shell/themes/compile.nix and can import Base16 palettes through themes/from-base16.nix; this spike extends that model outward to the rest of the desktop.

Checklist (from the planning session)

• Inventory every current Stylix consumer in ~/dotfiles (terminal, editor, GTK/Qt, browser, bar, notifications, wallpaper) and what each actually reads.
• Define the canonical rich theme format (palette + semantic roles + per-app overrides), versioned alongside schemas/theme.v2.json.
• Per-app adapters that render the canonical theme into each consumer's native config, replacing the Stylix targets one at a time.
• Base16 importer so existing schemes remain usable (from-base16.nix is the seed).
• Incremental migration: keep Stylix enabled for unmigrated targets; flip each target as its adapter lands; remove Stylix last.

Non-goals

Plugin API changes, icon migration, or any Stillsuit core work — those are tracked separately.