refactor(frontend): split lib/api.ts by domain

[attson]

Summary

Milestone 7-c. `api.ts` was 1098 lines wrapping ~140 `window.go.main.App` calls in one file: every domain (relay, ssh, feishu, tasks, updates, recovery) lived side-by-side with generic prefs and session lifecycle. Anyone touching one relay method had to skim 1000 unrelated lines to find it.

Split into a domain subdir:

```
lib/api/
_bindings.ts — AppBindings interface + Window decl + bindings() +
every wire type (moved from api.ts so domain files
can reference them without an api.ts import cycle)
relay.ts — 20 fns: config, login, register, me, pairing,
diagnostics, sessions listing, uplink health
ssh.ts — 9 fns: connect + hosts + keys
feishu.ts — 10 fns: mode + status + credentials + hook install
tasks.ts — 10 fns: preset/groupBy/sidebar + pins + templates +
mark-seen + broadcast-command-finished
updates.ts — 10 fns: updater state + download + install +
GH proxy + auto-check toggle
recovery.ts — 5 fns: load/save/discard + dialog-enabled toggle
api.ts — barrel `export *` for the 6 slices + "everything
else" (session lifecycle, generic prefs, notif
wrappers, logging, WebGL/shell-integration toggles)
```

Every existing consumer keeps `import { X } from "./lib/api"` unchanged because api.ts re-exports everything. No caller was renamed.

Sizes:

• api.ts: 1098 → 299 (-73%)
• api/*.ts: ~950 new lines across 7 files (mostly the shared `_bindings.ts`)

Test update: `App.theme.test.ts` scanned `api.ts?raw` for two AppBindings signatures (`"GetTerminalTheme(): Promise"`). After the split those live in `api/_bindings.ts`; the test now imports both files and scans the right one for interface signatures vs wrapper functions.

Test plan

• `npm test` 1631/1631
• `npm run build` (vue-tsc + vite) green
• Manual: grep all 38 consumers of `./lib/api` — all still typecheck; behavior unchanged (barrel forwards)