Pin folders in the session drawer so they don't reshuffle on every session switch

[goyamegh]

Summary

Let users pin folders (cwd groups) in the session drawer so they stay at a fixed position and don't reshuffle every time the user switches sessions.

Today, folders are ordered implicitly by the most-recent activity inside them — when you switch to a session in folder X, folder X bubbles up; when you flip to a session in folder Y, folder Y bubbles up. For users who actively rotate between many folders this means the drawer is in motion almost constantly and "muscle memory" for finding a folder never has a chance to form.

Why this matters

• Spatial stability beats recency for navigation. The drawer is the workspace map. Ordering by recency makes it a log instead of a map. Most navigation in the drawer is not "go to where I just was" (the current-session highlight already covers that); it's "go to that other folder I keep coming back to". A stable position makes that scan a sub-second visual pattern-match instead of a fresh read every time.
• Pinned tabs already exist for sessions. There's a strong precedent in the same drawer: pinned tabs / pinned sessions stay put while everything else moves around them. Pinned folders are the same idea one level up the hierarchy.
• Compounds with marker buckets. When you filter by a marker color, the surviving folders also reshuffle (see #13 for the related papercut). Pinned folders give the drawer two stable axes — color filter for "what to show" and pin order for "where to show it".

Proposed UX

• Each folder header gets a small pin/unpin affordance (icon button), mirroring the existing per-session pin control.
• Pinned folders sort to the top of the drawer in the order they were pinned, above all unpinned folders.
• Within the pinned region, order is stable — pinning a folder appends to the end of the pinned list, the user can drag to reorder, but switching sessions never reorders pinned folders.
• Unpinned folders below continue to use the existing recency-based ordering.
• Pin state persists per-device. (Whether to also sync via the existing pi-web-session-ui-state.json mechanism is up to the maintainer; either works.)
• A small visual treatment on pinned folder headers (e.g. a subtle accent border or a filled pin icon) makes the two regions distinguishable at a glance.

Where it would live

• src/sessions/sessionDrawer.ts — extend the folder grouping pass to partition folders into pinned[] and rest[], then concatenate. The grouping pass currently iterates visibleSessions (~line 1140); a small additional pass over a pinnedFolderCwds set is enough.
• Storage: extend pi-web-session-ui-state.json with a pinnedFolders: string[] (cwds, in pin order), or use localStorage if the maintainer prefers per-device.
• Server: a tiny POST /api/session-ui-state patch (already exists for pinned sessions / markers) to round-trip the field.
• Reuse the existing bookmark / pin icon set; no new icons needed.

Edge cases

• Renamed / removed folder: if a pinned cwd no longer has any sessions, render a placeholder ghost row (related to #13 which already argues for keeping empty folders visible) so the user can unpin it. Alternatively, auto-unpin after N days of zero activity.
• Pinned folder under a non-matching color filter: still render the folder header (consistent with #13) — pinning shouldn't be undone by an active filter.
• Drag-to-reorder: nice-to-have, not required for v1. Add-pin order is fine for an MVP.

Non-goals

• Folder grouping by anything other than cwd (e.g. by repo, by branch). Out of scope here.
• Nested folders / tree view. Out of scope.

Acceptance criteria

• Each folder header has a pin/unpin affordance.
• Pinned folders render above unpinned folders in a stable order across session switches.
• Pin state survives reloads.
• Pinning is independent of the active marker-color / search filter (a pinned folder remains pinned even when its sessions are all filtered out).
• No regressions to the existing per-session pin behaviour.

Related

• #13 — keep folders visible under filters / empty states.
• #15 — open the drawer on new-session creation (also about preserving spatial perspective).