feat(mobile): add measured cold-start home transitions

[wgqqqqq]

Summary

Authenticated cold process launches previously went straight to the mobile shell. Add a 2.4-second native transition on Android, iOS, and HarmonyOS: a small contour logo moves to the actual measured home-logo bounds, then the cover fades to reveal the page.

Keep the existing first-install reveal and signed-out welcome experience. Background resume, activity/root recreation, and later manual login do not replay the short transition. No per-version animation variant is introduced.

Type and Areas

Type: UI/UX, feature

Areas: Native mobile Android/Compose, iOS/SwiftUI, HarmonyOS/ArkUI, mobile design-system contracts

Motivation / Impact

The measured anchor follows native safe areas and home layout rather than a fixed target coordinate. The home stays mounted so remote restoration can continue underneath; the animation does not wait for remote network readiness. Missing anchors fall back to a centered fade. Reduced motion skips playback, and the covered shell is hidden from accessibility. Android cover/home sweep phases are synchronized for the handoff.

Verification

Completed in the implementation checkout before applying the changes cleanly onto current upstream/main:

• Android: ./gradlew :app:assembleDebug passed; ./gradlew :app:compileDebugKotlin passed again after the sweep synchronization change.
• HarmonyOS: hvigorw --mode module -p product=default -p module=entry@default assembleHap --no-daemon passed; debug HAP installed on a connected phone.
• HarmonyOS isolated cold-start-home / cold-start-home-dark previews: compact phone screenshots checked for logo landing and home fade in both themes; normal EntryAbility restored afterward. The fixture uses the production transition and recent-home component without account or remote requests.
• iOS: ./Testing/run-pure-swift-tests.sh passed. Swift syntax parsing passed for the changed app/animation entry points. This does not compile SwiftUI; full Xcode is unavailable on the test machine.
• pnpm run mobile:ui:check, pnpm run harmony:architecture, and git diff --check passed in the implementation checkout.

On the PR worktree based on updated upstream/main, generated mobile contract consistency, HarmonyOS architecture, and diff whitespace checks pass. The full preview asset check cannot complete in this fresh worktree because the canonical design-token CSS build and workspace dependencies are absent.

Known limitations:

• pnpm run mobile:architecture reports existing violations in unchanged shared-layer files: ClientBuildContractTest, AccountUiState, RemoteSidebarPresentation, and ConversationModels.
• iOS native build/device, wide/foldable postures, and the full authenticated/background lifecycle matrix remain unverified.
• No real remote-control, remote-workspace, peer-device, or detached-dispatch scenario was exercised; device checks are isolated presentation coverage only.

Reviewer Notes

Existing installation-local reveal preferences and account/transport payloads are retained. New process eligibility and measured geometry are presentation state. The HarmonyOS preview launch commands are documented in its local AGENTS.md.

Checklist

• This PR is focused and does not include secrets, temporary prompts, generated scratch files, or unrelated artifacts.
• Relevant verification is recorded above, or skipped checks are explained.
• User-facing strings, docs, and locales are updated where applicable.