fix(web): reconcile session lifecycle and surface hierarchy

[GCWing]

Summary

• make FlowChatStore the single session-selection owner and reconcile the Session scene for archive, delete, workspace removal, and device-surface changes
• improve workspace session batch management and destructive confirmations while keeping archive actions immediate and recoverable
• align startup and runtime window controls, native dragging boundaries, and maximize or restore state
• strengthen built-in theme chrome, scene, and elevated-surface hierarchy and refresh generated appearance projections

Type and Areas

Type: bug fix, UI/UX, refactor, test, docs

Areas: web UI, desktop/Tauri, appearance and theme governance, session projection docs

Motivation / Impact

Prevents orphaned Session scenes and stale presentation state after sessions disappear, makes session management available consistently from workspace menus, and improves visual separation across named themes and startup window chrome.

Verification

• PASS: pnpm run check:web
• PASS: pnpm run theme:color-audit:all
• PASS: pnpm run i18n:audit
• PASS: focused Web UI Vitest suite covering session lifecycle, activation and sync, scene top bar, startup preload, workspace/session actions, layout, appearance output, and surface recipes (12 files, 128 tests)
• PASS: cargo build -p openbitfun-desktop (existing MSVC LNK4098 warning only)
• PARTIAL: cargo test -p openbitfun-desktop --lib startup_window_control_contract_exposes_the_native_maximize_state compiled and linked, but the local Windows test binary exited before the harness with STATUS_ENTRYPOINT_NOT_FOUND; no Rust assertion failure was reported
• Not run: browser or E2E visual validation, per repository guidance; CI remains the merge gate

Remote scenarios: local desktop/Web UI paths were exercised. Peer Device Mode surface switching is covered by the focused state contract. Remote workspace, remote-control transports, and Detached Dispatch were not exercised; this change does not alter their wire contracts or persistence shapes.

Reviewer Notes

The branch is rebased onto the current origin/main. Generated startup and appearance snapshots are included with their canonical palette changes.

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.