Refine Foundry UI layout and styling (#235)

* feat: modernize chat UI and rename handoff to task

- Remove agent message bubbles, keep user bubbles (right-aligned)
- Rename "Handoffs" to "Tasks" with ListChecks icon in sidebar
- Move model picker inside composer, add renderFooter to ChatComposer SDK
- Make project sections collapsible with hover-only chevrons
- Remove divider between chat and composer
- Update model picker chevron to flip on open/close
- Replace all user-visible "handoff" strings with "task" across frontend

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

* feat: real org mock data, model picker styling, project icons, task minutes indicator

- Replace fake acme/* mock data with real rivet-dev GitHub org repos and PRs
- Fix model picker popover: dark gray surface with backdrop blur instead of pure black
- Add colored letter icons to project section headers (swap to chevron on hover)
- Add "847 min used" indicator in transcript header
- Rename browser tab title from OpenHandoff to Foundry
- Reduce transcript header title font weight to 500

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

* feat: refine Foundry UI — single-line task cards, dark user bubbles, curved panel corners, send icon

- Collapse task sidebar cards to single-line layout (title, number, diffs, timestamp)
- Dark-themed user message bubbles matching site theme
- Curved top-left corner on center chat panel with border line
- Subtle focus border on composer input
- Replace ArrowUpFromLine with SendHorizonal icon
- Tab strip gaps, padding, and divider alignment fixes
- Plus button with visible background
- Right sidebar header color matching

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

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Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

factory/packages/frontend/src/components/mock-layout/ui.tsx

@@ -178,8 +178,8 @@ export const Shell = styled("div", ({ $theme }) => ({
   display: "grid",
   gap: "1px",
   height: "100dvh",
-  backgroundColor: $theme.colors.borderOpaque,
-  gridTemplateColumns: "280px minmax(0, 1fr) 380px",
+  backgroundColor: $theme.colors.backgroundSecondary,
+  gridTemplateColumns: "minmax(0, 1fr) minmax(0, 1.5fr) 380px",
   overflow: "hidden",
 }));