Pinned folders v2 — file clusters via drag-drop

Issue: #189 — follow-up to #187 (which introduced pinned folders as activity-bar clusters) and PR #227 (the v1 drag groundwork).

What v2 adds

v1 (#227) wired up the plumbing — PinnedFolder.files: string[], the application/x-mid-file mime carried by file-tree drags, and a drop handler on each pinned activity icon. But the user-visible behavior was incomplete:

1. The drop highlight was identical for "drop file here" and "reorder pin", so users couldn't tell which gesture they were making.
2. When a pin had assigned files, the sidebar showed only those files and hid the folder subtree — there was no way to see both at once or drag another file in from the same folder while in cluster view.
3. The right-click menu on a clustered file said "Remove from cluster" instead of the spec-mandated "Remove from pin".

v2 closes those gaps. No schema migration: the files_json column on pinned_folders (added in v1) already stores the array. Persistence still flows through mid:patch-app-state → replacePinnedFolders in apps/electron/db.ts.

How it works

Drag a file from the tree onto a pinned activity icon

1. The file-tree row is the drag source. Each file <div class="mid-tree-item"> sets dataTransfer.setData('application/x-mid-file', entry.path) in its dragstart handler (renderer.ts renderTreeEntry).
2. The pinned activity-bar buttons are drop targets. On dragenter/dragover the handler peeks at dataTransfer.types (because getData() returns "" during drag-over for security reasons) and toggles one of two CSS classes:
   • .is-file-drop-target — dashed accent ring + 18% accent tint, set when the drag carries application/x-mid-file.
   • .is-drop-target — solid accent fill (the v1 behavior), set when the drag carries application/x-mid-pin (pin reorder). The handler also flips dataTransfer.dropEffect to 'link' for files and 'move' for reorder so the system cursor matches the gesture.
3. On drop, if application/x-mid-file is present we append the path to pin.files (deduped via Set), persist with window.mid.patchAppState({ pinnedFolders }), flash a toast, and re-render the cluster sidebar if it's currently open. If the file was already in the cluster the toast says so and the array is left alone.
4. If only application/x-mid-pin is present we fall through to the v1 reorder path.

Hybrid sidebar display

loadPinnedTree(folderPath) now renders two sections when a pin has assigned files:

If pin.files is empty the section header disappears and the sidebar reverts to the v1 folder-only listing. If both the cluster and the folder subtree are empty we still show the "No markdown files in this folder." empty state from v1.

Cluster file rows reuse the standard .mid-tree-item look with one extra class (.mid-tree-item--cluster) that hides the leading chevron so file names align with chevron-padded folder rows below.

Right-click on a cluster file row

Three actions, in order:

• Open — same as a click.
• Reveal in Finder — opens the parent directory.
• Remove from pin — strips the file path from pin.files, persists, and re-renders. The pin itself is untouched.

Where the code lives

Concern	File	Notes
Drag source on file rows	apps/electron/renderer/renderer.ts renderTreeEntry	application/x-mid-file (unchanged from v1)
Drop target on activity-bar pins	apps/electron/renderer/renderer.ts renderActivityPinned	dragenter/dragover/dragleave/drop
Cluster sidebar render	apps/electron/renderer/renderer.ts loadPinnedTree + renderClusterFileRow
Drop-target visuals	apps/electron/renderer/renderer.css .mid-activity-btn.is-drop-target / .is-file-drop-target
Cluster section visuals	apps/electron/renderer/renderer.css .mid-tree-section / .mid-tree-section-header / .mid-tree-item--cluster
Persistence	apps/electron/db.ts replacePinnedFolders (writes files_json); apps/electron/main.ts writeAppState / readAppState	unchanged from v1

Manual smoke test

1. Open a workspace folder with a few .md files.
2. Right-click a folder in the tree and pick Pin to sidebar…, accept defaults. The pin appears in the activity bar.
3. Drag a .md file from the file tree over the new pin icon. The icon should show a dashed accent ring (file-drop), not the solid fill.
4. Drop. A toast confirms the assignment.
5. Click the pin in the activity bar. The sidebar should show a Pinned files (1) section above a Folder contents section.
6. Drag a different pin's icon over another pin. The hovered pin should show the solid accent fill (reorder), not the dashed ring.
7. Right-click a clustered file row → Remove from pin. Section count updates. If it was the last file, the section disappears.
8. Quit the app, reopen, click the pin — the assigned files are still there (persisted in mid.sqlite pinned_folders.files_json).