refactor(frontend): fold main.ts / main.web.ts / main.capacitor.ts into bootstrapApp

[attson]

Summary

Milestone 7-d. The three Vite entry points open-coded the same six-step boot sequence:

1. `await initI18n({ loadPreference, savePreference })`
2. `bridgeSharedI18n()`
3. platform-specific pre-mount setup
4. `initPlatform()`
5. `createApp(App).use(Pinia).provide('platform').mount('#app')`
6. platform-specific post-mount side effects

Steps 1, 2, 4, 5 were identical text. Steps 3 and 6 were the only things that actually differed (Wails: `EventsOn` wire; web: prefsSync engine wiring; capacitor: iOS keyboard accessory + capacitor prefsSync + locale storage helpers). Three copies of the same boilerplate had already drifted — the web entry silently missed the shared-i18n locale bridge for a stretch, capacitor forgot the `prefs:changed` hook that Wails installs, etc.

Fold the common path into `lib/bootstrapApp.ts` with a `bootstrapApp({ i18n, createPlatform, beforeMount?, afterMount? })` API. Each main.* becomes a small config object naming what actually differs:

file	before	after
main.ts	35	21
main.web.ts	42	26
main.capacitor.ts	65	50
bootstrapApp.ts	—	+66

Behavior is preserved verbatim per entry — same load/save locale hooks, same platform factory, same prefsSync engines wired at the same point, same `EventsOn` hook, same iOS keyboard toggle. The shared-i18n locale bridge, previously a copy in each file, now runs unconditionally in `bootstrapApp` (matches what all three were already doing).

Test update: `App.test.ts > capacitor uses the shared web shell` was scanning main.capacitor.ts for `import App from './App.vue'` + `createApp(App)`; both moved into bootstrapApp.ts. The test now checks that capacitor main calls `bootstrapApp(` with `createCapacitorPlatform` and never re-introduces the old MobileApp shell.

Test plan

• `npm test` 1631/1631
• `npm run build` (vue-tsc + vite) green
• Manual: `wails dev` boots (Wails path), `npm run dev` boots (web path), Capacitor build syncs (iOS path)