feat(desktop): reorder pinned sessions by dragging within the sidebar

[SHL0MS]

What does this PR do?

Two related asks for the pinned-sessions sidebar: reorder pinned chats, and let the pinned section grow past ~6 rows.

Reorder (the substantive half)

Reordering pinned rows was effectively broken. The pinned list had a reorder affordance on a near-invisible dnd-kit grab handle (the leading status dot), but commit ac4f596ca2 made the whole row body a pointer session-drag source (drag a row onto the chat to link it as an @session ref, or onto a zone edge to split). The two drag systems fought on the same row and session-drag won: grabbing a pin started a link-into-chat drag, and even when you found the handle its dotted focus ring appeared but the reorder never committed.

Fixed by resolving the conflict the way the session-drag engine already works — by drop location, not by which pixel you grab. The engine is a resolver: it snapshots targets at drag engage, hit-tests the pointer each move, and acts on release. This adds sidebar reorder as one more resolved target:

• reorder-zones.ts — a small registry a flat list joins while mounted (container element + live id getter + onReorder). The drag snapshots registered zones at engage; resolveReorderTarget picks the insertion slot by pointer-Y (skipping the dragged row); reorderIds applies it with a no-op-identity guard. Both are pure functions, unit-tested.
• session-drag.ts — resolveMove checks reorder first (inside the bar's bounds it wins), otherwise falls through to the existing stack/split/link zone logic unchanged; onCommit reorders in place instead of opening a tile or link. A dedicated $sidebarReorderHint atom drives the insertion line so the chat surfaces never re-render on reorder churn (they subscribe to the heavy $dropHint only for real link/split hints).
• reorder-zone-list.tsx — registers the zone, tags rows (data-reorder-row) for the geometry snapshot, and paints the insertion line. No dnd-kit handle. The pinned section renders through it (reorderViaDrag) instead of ReorderableList.

Net behavior: drag a pin within the sidebar → reorder; drag it onto the chat → link (unchanged). Recents/Sessions rows keep session-drag exactly as before. This is the model the reporter asked for and it matches how the engine was built to route.

Height

The pinned body was hard-capped at max-h-44 (~6 rows) then scrolled. Relaxed to max-h-[40vh] so more pins show on a tall window. Still bounded, so the Sessions section below can never be buried — preserving the #43147 anti-overlap invariant — and GROUP_BODY still flattens the cap in compact mode.

Related Issue

Fixes the reorder half of #47728 (closed implemented-on-main, but reorder never actually worked once row-body session-drag landed — the handle was shadowed by the link drag). The height cap is the other half of the reporter's ask.

Type of Change

• 🐛 Bug fix (non-breaking change that fixes an issue)
• ✨ New feature (the height relaxation + drop-location routing)

How was this tested?

• Unit (reorder-zones.test.ts, 11 cases): slot-boundary math, dragged-row skip, foreign-drop rejection (a non-pinned row over the zone is not a reorder), multi-zone hit-testing, and reorderIds no-op identity + end-insertion.
• Component (reorder-zone-list.test.tsx, 4 cases): zone register/unregister lifecycle, live-id reporting across re-render, per-row tagging, and insertion-line rendering only for a drag of the list's own row.
• Real pointer-drag e2e (pinned-reorder.spec.ts): creates two pinned sessions, drags the top row down within the sidebar via actual mouse.down/move/up (stepped, like tile-unread-bug.spec), asserts the pinned order flips. Ran locally against the packaged macOS app — passes (1 passed (7.7s)).
• npx tsc -p . / tsconfig.electron.json / tsconfig.e2e.json all clean; full renderer suite 2050 passed; eslint clean on touched files (test-file document warnings match existing test conventions).

Checklist

Code

• I've read the Contributing Guide
• My commit messages follow Conventional Commits
• I searched for existing PRs to make sure this isn't a duplicate
• My PR contains only changes related to this fix/feature
• I've run the desktop renderer suite (npx vitest run --project ui) and all tests pass; pytest N/A (no Python changes)
• I've added tests for my changes (unit + component + real-drag e2e)
• I've tested on my platform: macOS 26.5 (arm64), packaged app

Documentation & Housekeeping

• Docs — N/A (no user-facing doc describes the pinned drag/height behavior)
• cli-config.yaml.example — N/A (no config keys)
• CONTRIBUTING.md / AGENTS.md — N/A
• Cross-platform impact considered — renderer + shared drag engine; the max-h-[40vh] and pointer-drag path are platform-agnostic. The e2e was exercised on macOS.
• Tool descriptions/schemas — N/A

[alt-glitch]

This was generated by AI during triage.

Related: #43661 also changes Desktop sidebar drag/reorder behavior, while #66051 handles persisted manual pin order. This PR resolves the row-drag conflict through drop-location routing with separate reorder zones; the overlapping but different mechanisms need maintainer consolidation.

[Nicolas-Formenton]

FYI: opened #84984 (drag to reorder profile groups in the All-profiles sidebar). It also touches sessions-section.tsx/index.tsx but only the profile-mode groups branch, disjoint from the pinned-session path here — no behavioral overlap, likely trivial import-context conflicts if either merges first.