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>

---------

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

factory/packages/frontend/src/components/mock-layout/transcript-header.tsx

@@ -30,7 +30,7 @@ export const TranscriptHeader = memo(function TranscriptHeader({
   const [css, theme] = useStyletron();
 
   return (
-    <PanelHeaderBar>
+    <PanelHeaderBar $style={{ backgroundColor: "#0f0f11", borderBottom: "none" }}>
       {editingField === "title" ? (
         <input
           autoFocus
@@ -58,7 +58,7 @@ export const TranscriptHeader = memo(function TranscriptHeader({
         <LabelSmall
           title="Rename"
           color={theme.colors.contentPrimary}
-          $style={{ fontWeight: 500, whiteSpace: "nowrap", cursor: "pointer", ":hover": { textDecoration: "underline" } }}
+          $style={{ fontWeight: 400, whiteSpace: "nowrap", cursor: "pointer", ":hover": { textDecoration: "underline" } }}
           onClick={() => onStartEditingField("title", handoff.title)}
         >
           {handoff.title}