Chat sidebar: nest sessions into a parent→children tree (drag-and-drop)

[arsenmuk]

Adds a parent→children hierarchy to the chat session sidebar, built on the existing sessions.parent_session_id column (no schema change).

What you can do

• Drag a session onto another to nest it as a child — the drop-target row highlights while hovering.
• A parent shows an expand/collapse chevron in place of its icon; children render indented beneath it, to any depth. When collapsed it badges its hidden direct-child count (mirrors the group-header counters).
• Nesting appears wherever the parent renders, including the pinned Starred group. Archived / System sessions stay only in their own groups.
• Un-parent two ways: the row's ⋯ menu → "Remove from parent", or drop a nested row onto the "remove from parent" strip that appears while dragging one (native event-bubbling, no position math).
• Expand state persists in localStorage; the active session's ancestors auto-expand so it's never hidden. Existing forks (which already set parent_session_id) nest automatically.

Server

• PATCH /api/sessions/{id} now accepts parent_session_id (null clears it → top-level), guarded against self-parent, unknown parent, cycles, and the created fork window so a display drag can never turn a not-yet-started session into a fork.
• The field already rides in the feed payload, so an unlimited page size renders the full tree; the client only draws what the server returns (an orphan whose parent isn't in the payload falls back to top-level).

Drag-and-drop uses native HTML5 DnD (no new dependency). Tests: tests/test_session_parent_patch.py (set / clear / self / unknown / cycle / fork-window / deep-chain); frontend tsc -b + build clean.

Stacked on #269 (its branch is this PR's base until it merges).

🤖 Generated with Claude Code

[Copilot]

Pull request overview

Adds hierarchical, drag-and-drop session nesting to the chat sidebar using the existing parent relationship.

Changes:

• Renders recursive, collapsible session trees with persisted expansion state.
• Adds optimistic reparenting and unparenting controls.
• Extends the PATCH endpoint with parent validation and tests.

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
web/src/utils/dateGroups.ts	Persists expanded parent IDs.
web/src/stores/chatStore.ts	Adds optimistic parent updates.
web/src/components/Chat/SessionSidebar.tsx	Implements nested rendering and drag-and-drop.
web/src/api/client.ts	Extends session update typing.
tests/test_session_parent_patch.py	Tests parent PATCH behavior.
nerve/gateway/routes/sessions.py	Validates and saves parent relationships.

Suppressed comments (2)

nerve/gateway/routes/sessions.py:441

• The cycle check and the update are not atomic. Concurrent requests for A→B and B→A can both read parentless rows, both pass this check, and then have their separately locked updates commit a cycle. Perform ancestor validation and the update under one database write lock/transaction, and cover the concurrent case.

            if await _would_create_cycle(deps.db, session_id, new_parent):
                raise HTTPException(status_code=400, detail="That drop would create a cycle")

web/src/components/Chat/SessionSidebar.tsx:1065

• HTML5 draggable provides no keyboard operation for assigning a parent, so keyboard-only users cannot use the core nesting feature. Add an accessible non-drag action such as “Move under…” in the row menu, or equivalent keyboard controls with announced grab/drop state.

      draggable={draggable || undefined}
      onDragStart={draggable && dnd ? (e) => {
        // Override the browser's default <a href> drag with our session id.
        e.dataTransfer.setData('text/plain', session.id);
        e.dataTransfer.effectAllowed = 'move';
        dnd.onDragStart(session.id);
      } : undefined}

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