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/prompt-composer.tsx

@@ -1,7 +1,7 @@
 import { memo, type Ref } from "react";
 import { useStyletron } from "baseui";
 import { ChatComposer, type ChatComposerClassNames } from "@sandbox-agent/react";
-import { ArrowUpFromLine, FileCode, Square, X } from "lucide-react";
+import { FileCode, SendHorizonal, Square, X } from "lucide-react";
 
 import { ModelPicker } from "./model-picker";
 import { PROMPT_TEXTAREA_MAX_HEIGHT, PROMPT_TEXTAREA_MIN_HEIGHT } from "./ui";
@@ -45,7 +45,7 @@ export const PromptComposer = memo(function PromptComposer({
       borderRadius: "16px",
       minHeight: `${PROMPT_TEXTAREA_MIN_HEIGHT + 36}px`,
       transition: "border-color 200ms ease",
-      ":focus-within": { borderColor: "rgba(255, 255, 255, 0.3)" },
+      ":focus-within": { borderColor: "rgba(255, 255, 255, 0.15)" },
       display: "flex",
       flexDirection: "column",
     }),
@@ -82,9 +82,9 @@ export const PromptComposer = memo(function PromptComposer({
       justifyContent: "center",
       color: theme.colors.contentPrimary,
       transition: "background 200ms ease",
-      backgroundColor: isRunning ? "rgba(255, 255, 255, 0.06)" : "#ff4f00",
+      backgroundColor: isRunning ? "rgba(255, 255, 255, 0.06)" : "rgba(255, 255, 255, 0.12)",
       ":hover": {
-        backgroundColor: isRunning ? "rgba(255, 255, 255, 0.12)" : "#ff6a00",
+        backgroundColor: isRunning ? "rgba(255, 255, 255, 0.12)" : "rgba(255, 255, 255, 0.20)",
       },
       ":disabled": {
         cursor: "not-allowed",
@@ -157,7 +157,7 @@ export const PromptComposer = memo(function PromptComposer({
         allowEmptySubmit={isRunning}
         submitLabel={isRunning ? "Stop" : "Send"}
         classNames={composerClassNames}
-        renderSubmitContent={() => (isRunning ? <Square size={16} /> : <ArrowUpFromLine size={16} />)}
+        renderSubmitContent={() => (isRunning ? <Square size={16} /> : <SendHorizonal size={16} />)}
         renderFooter={() => (
           <div className={css({ padding: "0 10px 8px" })}>
             <ModelPicker