feat(tabs): implement maximize/reset-split chord shortcuts + cross-group tab movement

[h4yfans]

Summary

Part of Phase 5 (UI stubs + polish) in the tech-debt-remediation plan — see .claude/plans/tech-debt-remediation.md § 5.1.

Closes four TODOs in apps/desktop/src/renderer/src/hooks/use-chord-shortcuts.ts (lines 90, 104, 128, 133).

Reducer changes

• TOGGLE_MAXIMIZE_GROUP: snapshots state.layout into preMaximizeLayout, collapses the tree to a single leaf on the active group, and sets isMaximized: true. Second dispatch restores from the snapshot.
• RESET_SPLIT_RATIOS: recursively rewrites every SplitLayout node's ratio to 0.5.

State changes

• TabSystemState.isMaximized?: boolean
• TabSystemState.preMaximizeLayout?: SplitLayout

Hook changes

• ⌘K ⇧→ / ⌘K ⇧← — move the active tab across adjacent groups (wraps) via existing MOVE_TAB action.
• ⌘K M — dispatch TOGGLE_MAXIMIZE_GROUP.
• ⌘K O — dispatch RESET_SPLIT_RATIOS.

Tests

• Reducer unit tests for both new actions (including idempotent restore and nested tree resets).
• New use-chord-shortcuts.test.tsx covering the four chord paths with a mocked useTabs context.
• Extended tabs.e2e.ts Playwright spec exercising cross-group move + maximize + reset in a real Electron window.

Test plan

• pnpm exec tsc -p tsconfig.web.json --noEmit clean in the worktree
• CI: pnpm test --filter @memry/desktop green on reducer + hook tests
• CI: pnpm test:e2e --grep chord green against a built bundle
• Manual smoke: open two tab groups in pnpm dev, exercise ⌘K ⇧→, ⌘K M, ⌘K O