perf(web): split heavy UI and guarded Tauri modules

[Tutitoos]

Summary

• lazy-load the admin surface, conversation reader, and composer so the authenticated inbox does not pay their startup cost
• load Tauri core only inside guarded desktop execution paths while preserving command arguments and error behavior
• add localized loading feedback for the deferred composer
• reduce the production entry chunk from 743.45 kB to 388.55 kB (47.7%) and remove the prior >500 kB and ineffective dynamic-import warnings

Validation

• bun run --cwd apps/web test (50 passed)
• bun run --cwd apps/web typecheck
• bun run --cwd apps/web build
• go test ./internal/modules/translations
• bun run --cwd apps/web test:e2e (29 passed; one desktop recovery navigation race)
• isolated retry: bunx playwright test tests/bootstrap.spec.ts --project=desktop --grep 'offers localized passkey' (1 passed)

Closes #192