feat: project groups (collapsible folders in the sidebar project list)

[MHohlios]

Closes #76.

What

Projects can carry an optional group label. Grouped projects render under collapsible folder headers in the sidebar, so many similarly named repos (app-ios / app-android / service-auth / ...) can be clustered by product or layer while staying visible at a glance. Groups are purely derived: a group exists while at least one project carries its label. No group entity, no filesystem effect, and existing projects.json files load unchanged.

UX

• Right-click a project: Move to group (existing groups / New group / Remove from group), works in compact mode too
• Drag and drop: drag a project into, out of, or within a folder; drop on a collapsed folder's header to file it; drag a folder header to reorder whole groups as blocks
• Group names are ALL-CAPS by design (the rename input enforces it; matching is case-insensitive so "frontend" and "Frontend" cannot split into two groups)
• Folder colors: the folder context menu opens with a Finder-tag-style inline swatch row (Default + 8 fixed colors). Color paints the folder glyph, name, and the members' guide line. Default folders stay muted (fg-faint) so only assigned colors are loud
• Collapsed folders roll up member agent state into a header dot (attention wins over done) so hidden members can still call for the user
• Header shows a member count; collapse state persists across launches
• "Expand all agents" / "Collapse all agents" also expand/collapse group folders
• Keyboard nav (option-arrows, next/prev task) walks the same visual order the sidebar renders
• A11y: headers are focusable buttons with aria-expanded; swatches carry aria-labels

Implementation

• src-tauri/src/lib.rs: optional group field on Project (serde default + skip-if-none) and one atomic project_set_group(ids, group) command so rename/dissolve is a single projects.json write
• src/lib/projectGroups.ts (new): groupOf() is the single normalization point (trim + uppercase); shared section/order helpers keep sidebar rendering and keyboard nav on one definition of visual order
• src/store/app.ts: collapse + color state in localStorage keyed by normalized group name, pruned when a group disappears, migrated on rename
• src/components/sidebar/Sidebar.tsx: folder rendering and the pointer-event drag logic (document-level listeners, arm-on-pointerdown, threshold start, live optimistic reorder)
• Folder accents come from a fixed palette table mapped to --color-palette-* tokens in @theme (light theme re-tunes the shades for contrast); everything else the feature paints uses existing theme tokens (fg-faint for default folders, warn/info for the roll-up dot, accent for drag highlights)
• Input hygiene: the stored color is a palette key validated against the table before rendering (unknown keys fall back to default), group names render only as React-escaped text and never reach a CSS value, the two interpolated selectors are CSS.escaped, and JSON-parsed localStorage maps are read with Object.hasOwn

Testing

• make check-all clean (cargo check + tsc), vitest 214/214, production build passes
• Manual pass on a fleet of test repos: grouping via menu and drag, folder reorder, collapsed-folder drops, rename merge, dissolve, collapse/expand all, compact rail, hide-inactive interaction, theme switching, relaunch persistence

Deliberately out of scope (follow-ups)

• New Project dialog: pick or create a group at add time
• Group header "+" action (choose what to create inside the folder)
• Theme-defined palette shades (per-theme swatch tuning + custom-theme keys) - prepared on a separate branch, PR to follow if wanted

[MHohlios]

Made some UX assumptions here so feel free to push back.

[adamatan]

Love it. Clean, easy to use.

Reordering works well.

Good improvement!

[simion]

Looks slick and clean! Congrats. Will test, review and merge tonight.