WS-1 foundation: gpui-kit boot, window, Root, and the light/dark/system theme

[LeadcodeDev]

Part of #306. Batch 1 — every other workstream depends on this one.

Replace the Dioxus desktop shell with a gpui-kit application: boot, window,
Root wrapper, and the theme system including the System mode the kit does
not provide.

Owned files

• src/app/root.rs (rewrite), src/app/window.rs (new)
• src/theme/mod.rs (new), src/theme/persist.rs (new)
• Deletes: src/components/** (13 dirs, 1 636 .rs + 2 440 .css),
  assets/dx-components-theme.css

Do NOT write: Cargo.toml, src/lib.rs, any mod.rs, src/app/state.rs
(orchestrator-owned — report the lines you need instead).

Deliverables

1. Boot — gpui_kit::application().run(...) + gpui_kit::init(cx).
   There is no Application::new(). Window sized to 75% of the current
   monitor, centered, as today.
2. Root — mandatory top-level wrapper. Root::render does not mount
   the dialog/sheet/notification layers; the root view must call
   Root::render_dialog_layer, render_sheet_layer,
   render_notification_layer itself.
   Isolate these three calls in one small file — 0.7.0 removes them with no
   documented replacement, and we want that migration to be a localised patch.
3. Theme — ThemePref { Light, Dark, System }. The kit's ThemeMode has
   only Light | Dark and theme::init hard-sets Light; nothing observes
   the OS. Wire it:

   Theme::sync_system_appearance(Some(window), cx);
   window.observe_window_appearance(|w, cx| Theme::sync_system_appearance(Some(w), cx)).detach();

   After mutating Theme::global_mut, call Theme::sync_base(cx) — otherwise
   the gpui-base layer (scrollbars, resize handles) keeps the old values.
4. Persistence — the preference is stored next to the recents, i.e.
   dirs::config_dir()/rustmotion/. Today the theme resets to System at
   every launch; it must survive a restart. Follow the shape of
   load_recents/push_recent in src/library/data.rs:233-266.
5. Deletion — the whole src/components/ tree. gpui-kit is used directly,
   with no wrapper layer. All 6 live components have a kit equivalent.

Notes that will save you time

• ActiveTheme must be imported from gpui_component, it is not re-exported
  at the kit root. Then cx.theme().background etc.
• The token is danger, not destructive, on ThemeColor.
• Map today's --rm-* tokens (defined in src/app/root.rs:28-79) onto the
  kit's semantic tokens. Note the current doc comment at line 25 contradicts
  the CSS: .rm-system is in the dark rule and swaps to light under
  prefers-color-scheme: light. The CSS is the behaviour to reproduce.
• Two IconName enums exist. gpui_component::IconName (101 variants) is what
  gpui_kit_assets::Assets actually embeds; using a gpui_kit_assets::IconName
  variant (1830 Lucide) while registering only Assets silently fails to load.

Verify

CARGO_TARGET_DIR=/tmp/rm-ws1 rtk cargo build -p rustmotion-studio
CARGO_TARGET_DIR=/tmp/rm-ws1 rtk cargo test -p rustmotion-studio
CARGO_TARGET_DIR=/tmp/rm-ws1 rtk cargo clippy -p rustmotion-studio --all-targets -- -D warnings