feat(app): sort sidebar projects

[graydawnc]

What

Three changes that grew out of the same iteration on the library shell.

1. Sidebar project sort menu

Hover-revealed sort icon next to the PROJECTS label opens a small menu with:

• Recent activity (default) — lastSessionAt DESC
• Name (A–Z) — locale-aware, case-insensitive localeCompare
• Most sessions — sessionCount DESC, ties broken by recency

Selection persists on AgentsConfig (sits alongside the existing sidebarShowSourceDots / sidebarShowSessionCount flags) so it survives restarts. Sorting happens in JS over the existing listProjectGroups payload — no DB query or IPC contract changes. The Loose ("no-git") group stays pinned to the bottom regardless of order.

2. Scroll fade-out on main surfaces

A mask-image: linear-gradient over the last 20–24px on every scrollable list:

• Sidebar project list
• LibraryLanding feed
• ProjectView session list
• SessionDetail message list

Half-clipped rows at the scroll edge now read as "more below, keep scrolling" instead of "broken edge".

3. Default window 1180×780 → 1080×740

The library redesign had stretched the default further than DESIGN.md's "compact utility, ~960px" stance. Brings it back closer to spec. Min sizes (800×520) unchanged so users can still resize.

Why

The sort menu is the gap most directly visible at scale: at hundreds of sessions across 30+ projects, recency is the only ordering and the tail of the list becomes unfindable without knowing roughly when something was last touched. Three options are enough — a symmetric fourth (First started) was considered and dropped because it has near-zero usage value at this granularity.

The fade-out and the window-size tweak fall out of the same observation: the sidebar (and main feed) at default size always shows N+½ rows, and there's no window height that fixes that for everyone (project counts vary, system DPI varies). Treating it as a visual-affordance problem — fade the edge — is robust across all sizes; treating it as a sizing problem only solves it for one specific row count. Pulling the default down rather than pushing it up matches the "compact utility" direction in DESIGN.md.

How it connects

• The sort options mirror parts of the ProjectView sort menu (Recent / Most messages / Title) but at project granularity. Wording and the ✓-active pattern match the existing Menu component.
• Sort persistence is wired through the existing getAgentsConfig / setAgentsConfig IPC. App owns local state and writes through on change; no new IPC handler.
• The Sidebar consumes status from props and now also sortOrder / onSortOrderChange, so the App is the single source of truth for sidebar pref state.
• The [mask-image:linear-gradient(to_bottom,...)] Tailwind arbitrary value works directly in Chromium (no -webkit- prefix needed).

Test plan

• tsc clean
• vitest 12/12 pass
• Manual: hover PROJECTS, sort icon appears, all three options change order
• Manual: choose Most sessions, restart — selection restored
• Manual: Loose group stays at the bottom across all sort orders
• Manual: bottom edge of all four scroll surfaces fades out
• Manual: default-launched window is 1080×740, sidebar still scrolls projects