feat: pin multiple projects with split workspace layout

[justguy]

Summary

Closes #2.

Projects can now be pinned from the overview drawer. Pinned projects render as side-by-side panes in the workspace; each shows its own header, scratchpad, progress strip, and matrix.

Behaviour

• Pin toggle in each drawer row: `[PIN]` / `[PINNED]`. Persisted to localStorage (same place as `theme` and `drawerPinned`).
• No pins → current single-pane view unchanged (renders `activeSlug`).
• 1 pin → the pane takes 50 % width, as the issue specified.
• 2+ pins → panes split the workspace evenly (`flex: 1 1 0` each).
• Minimum pane width 480 px. If total minimum exceeds the viewport, the workspace scrolls horizontally (`overflow-x: auto`).
• Click a pane to focus it — becomes the new `activeSlug`, so the top-bar `[UNDO]` / `[HISTORY]` / `[DELETE]` target it. Active pane gets an amber underline on its header.
• Shared across panes: theme, overview drawer, filter input text, status/block filter toggles (counts come from the active pane).
• Per-pane: scratchpad banner, error banner, progress strip, swimlane × priority matrix, all task interactions (drag, delete, comment edit).

Refactor

Callbacks that previously closed over `activeSlug` now take `slug` as the first argument:

callback	before	after
`onMove`	`onMove({...})`	`onMove(slug, {...})`
`onToggleCollapse`	`onToggleCollapse(lane, bool)`	`onToggleCollapse(slug, lane, bool)`
`onDeleteTask`	`onDeleteTask(task)`	`onDeleteTask(slug, task)`
`onSaveComment`	`onSaveComment(taskId, val)`	`onSaveComment(slug, taskId, val)`

`ProjectPane` binds its own slug when passing them to `Matrix`, so nothing internal to the card/cell/matrix tree changed.

Not in this PR

• Per-pane `[UNDO]` / `[HISTORY]` / `[DELETE]` buttons — header-level actions still operate on the focused pane only. Reasonable follow-up.
• Drag to reorder panes. Follow-up.
• Resizable dividers between panes. Follow-up.
• Per-pane status/block filters. Current behaviour: filters are shared, counts come from the focused pane.

Test plan

• `npm test` — 63/63 pass
• `node --check ui/app.js` — syntax clean
• Hub CLI still boots (`node bin/llm-tracker.js --help`)
• Manual: no pins → single-pane view unchanged vs. before this PR.
• Manual: pin one project → pane renders at 50 % workspace width.
• Manual: pin a second → both split 50/50, scratchpad shown per-pane.
• Manual: pin three or four → equal split, all readable; six+ → horizontal scroll kicks in.
• Manual: click an unfocused pane → becomes active (amber underline), top-bar undo/history/delete now target it.
• Manual: drag a card within a non-focused pane → task moves in the correct project (regression check on the callback refactor).
• Manual: edit a comment in a non-focused pane → saves against the correct slug.
• Manual: unpin all → returns to single-pane view on `activeSlug`.
• Manual: reload browser → pinned set persists.