feat(desktop): reorder sessions by dragging the session list

[czs914893385]

What

Adds manual ordering to the desktop session list: a new Manual session-view ordering mode and drag-and-drop reordering of the session rows in the left nav.

Why

The session list could only be sorted by updated / status / created / name, so sessions a user wants to keep together had no way to stay where the user put them. #3350 (merged) added the session-row menu action for copying a session; this uses the same surface, so a session row can now be copied or arranged by hand.

How

• New pure module src/web-ui/src/flow_chat/utils/sessionManualOrder.ts (move inside the stored list, reconcile it against the sessions that exist, order a view from a stored list) with its own tests.
• The existing session-view preference store gains a scope-keyed manualOrder map (workspace:<id> for one workspace group, all for the flat "all sessions" view) beside the existing ordering preference, persisted the same way. It is a local UI preference: corrupt or legacy values are normalized and never throw, and the persist version is not bumped.
• Ordering is applied at the single existing sort point in SessionsSection.tsx; every other ordering mode keeps its current behaviour.
• Drag uses native HTML5 drag events, the same pattern as the existing workspace drag reorder: custom MIME type, insertion side decided by the row's vertical midline, drop indicator, and the existing 160 ms FLIP settle that honours prefers-reduced-motion.
• Dragging works only within one scope. A drop from another scope or sibling group is rejected before any state changes.
• Committing a reorder switches the view to manual ordering, silently: dragging is a frequent gesture, so a notice per drop would be noise. The ordering control reflects the switch.
• Rows that are loaded but not rendered cannot take part in a reorder, and the stored order keeps slots for sessions that are not loaded, so dragging can never reorder a row the user cannot see.
• Sessions that disappear from the list are pruned from the stored order (only ids actually observed to vanish).

Limits

• The order is local to the device: no host-side field, no protocol change, no cross-device or mobile sync.
• Web UI only: no Rust changes and no new dependencies.

Testing

• pnpm run lint:web - 0 errors (the 10 remaining warnings are pre-existing in unrelated files).
• pnpm --dir src/web-ui run test:run - 972 files / 10760 tests pass, including the new sessionManualOrder.test.ts (17) and sessionRowManualReorder.test.tsx (17); the existing sessionRowContextMenu.test.ts (6) still passes.
• pnpm run build:web - green: appearance contract audit, parallel type-check + vite build, frontend revision, monaco asset verification, WebKit compatibility gate.
• node scripts/i18n-audit.mjs - clean.
• Manually verified in a locally built desktop preview: a real mouse drag with the drop indicator, the resulting order change, the silent mode hand-off, and persistence across a restart.

i18n

nav.sessions.viewMenu.ordering.manual added to en-US / zh-CN / zh-TW.

中文说明

• 会话列表新增「手动排序」模式，并支持在左侧列表里拖动会话行调整顺序。
• 顺序按作用域记忆（每个工作区分组一份，扁平「全部会话」视图一份），存在本机的界面偏好里，重启后保留；旧的或损坏的存储值会被规整，不会抛错。
• 拖动只在同一作用域内生效；跨分组拖动会被直接拒绝。
• 落地一次拖动会静默切换到「手动排序」（拖动很频繁，因此不弹提示），排序控件会显示当前模式。
• 已加载但未渲染（分页之外）的行不参与排序；存储顺序会保留未加载会话的位置，会话消失时只清理确实消失的 id。
• 局限：顺序只存本机，不做跨设备/手机端同步；纯前端改动，无 Rust 与依赖变更。

[czs914893385]

Closing this for now - I want to test the interaction locally before asking for review. Sorry for the noise.

[czs914893385]

Reopening: the lint failure in the previous revision is fixed, and the row-menu Move up / Move down items were dropped at the reporter's request, so this is now drag-only plus the Manual ordering mode.