fix(config): preserve settings across saves and account sync

[bobleer]

Summary

Fix settings loss across local saves, account synchronization, and backup restoration. A saved realtime voice API key could disappear when an older or unconfigured account snapshot replaced local settings. The same paths also exposed other settings and model credentials to stale read/write races.

• Preserve locally configured fixed fields omitted by older imports, and preserve a configured realtime voice key when account sync supplies an empty key. Explicit clears, resets, deleted list/map entries, and legacy migrations retain their intended behavior.
• Commit in-memory state only after persistence succeeds. Keep configuration read/modify/write operations under one lock, retain the storage root on reload, and give backups unique names with a pre-import recovery copy.
• Subscribe account sync to successful local configuration changes and reject a fetched cloud snapshot if local settings changed during the request. Cloud imports do not echo a local-change notification.
• Remove write-on-read model migration, serialize fresh-value frontend edits, and save only edited AI experience fields. Treat import failures as failures and keep an explicitly empty quick-action list empty.

Type and Areas

Bug fix / regression fix. Rust configuration services and account sync, Desktop adapters, Web UI settings, regression tests, and compatibility documentation.

Motivation / Impact

Saving another setting, restarting, restoring an older backup, or receiving a delayed sync response must not silently discard unrelated settings or credentials. This also prevents failed disk writes from being committed by a later successful save and prevents concurrent model changes from losing or resurrecting entries.

Verification

GitHub CI passed on PR head 4698bfc9232f82010983b76b1b1754bf0ffb8d92: 10 successful checks, with only the 2 unrelated packaging jobs skipped by the existing impact classifier. Rust/CLI checks passed on Linux, macOS, and Windows. The full Web UI suite passed all 4,286 tests across 591 files, and frontend build/governance checks passed. No CI rules were weakened and no reruns were needed.

Passed locally before rebasing onto the latest target branch; the intervening upstream commit changes unrelated Write-tool code. Rust commands used an isolated temporary Cargo target directory, and tests used synthetic temporary settings rather than the user's configuration.

• cargo test -p bitfun-core --no-default-features --lib service::config:: --quiet — 120 passed.
• cargo test -p bitfun-core --no-default-features --features remote-connect --lib service::config:: --quiet — 131 passed.
• cargo test -p bitfun-core --no-default-features --features remote-connect --lib service::remote_connect::settings_sync::tests --quiet — 4 passed.
• The focused frontend command below — 103 passed across 7 files.
• pnpm run check:web — passed.
• cargo test -p bitfun-desktop --lib api::config_api::tests --quiet — passed.
• cargo check -p bitfun-desktop --quiet and cargo build -p bitfun-desktop --bin bitfun-desktop --quiet — passed.
• pnpm run fmt:rs and git diff --check — passed.

After rebasing, the remote-connect configuration suite was rerun on the PR commit (131 passed), and pnpm run lint:web passed with no errors.

pnpm --dir src/web-ui run test:run \
  src/infrastructure/config/services/ConfigManager.test.ts \
  src/infrastructure/config/services/AIExperienceConfigService.test.ts \
  src/infrastructure/config/services/modelConfigs.test.ts \
  src/infrastructure/api/service-api/ConfigAPI.test.ts \
  src/app/services/agentCompanionPetCommands.test.ts \
  src/infrastructure/api/adapters/peer-device-adapter.test.ts \
  src/flow_chat/components/modern/SessionFilesBadge.test.tsx

Reviewer Notes

• AI-assisted with Codex. Testing level: focused automated regression tests and native build completed; manual UI and remote end-to-end testing were not performed.
• Remote coverage is limited to shared configuration/sync logic and Peer adapter contracts. Remote workspace, remote control, Peer Device Mode, and Detached Dispatch were not exercised on real devices.
• The Windows desktop library test binary hit the existing Tauri #13419 loader limitation before running tests. The repository's unchanged probe handled that known signature; Windows compilation, core/CLI tests, and portable contracts passed. Linux/macOS desktop library suites ran successfully.
• No new host RPC or persisted schema requirement is introduced. Missing fixed fields and explicit deletion/reset payloads have separate regression coverage; supplied dynamic maps and arrays remain authoritative.
• No visual layout changes. A rebuilt Rust backend is required to exercise the persistence and sync fixes.

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.