feat(web): restore polished workbench UI and conversation rendering

[somewan820]

Problem

The session-admission hardening inside #326 (commit d1118f8) rewrote web/ui and silently dropped most of the polished design from 4421944 (feat(web): polish workbench UI): the settings dialog (language/theme pickers), the landing logo animation, rich thinking/tool-call rendering, subagent/workflow status cards, the turn rail, the composer activity bar, the favicon, and the engineering-grid backdrop. The merged workbench shipped with a minimal generic transcript instead, far from the workbench experience outlined in #76.

Value

• Restores the reviewed 4421944 design baseline the web workbench was specified against ([Roadmap] OpenPI Web：像 Codex / Kimi Code 一样的本地浏览器 Agent 工作台 #76).
• Re-establishes the UI foundation several open web issues build on: theme/language preferences (feat(web): add persistent theme/language preferences and accessibility/mobile hardening #350), structured tool evidence rendering (feat(web): add purpose-built renderers for file, diff, test, and terminal evidence #345), and capability activity surfaces (feat(web): add inspectable and controllable OpenPI capability panels #346).
• Keeps every hardening fix from d1118f8 and later (session admission, event recovery, lease/terminal boundaries) — this is a UI restoration, not a revert.

Approach

• Restored web/ui/{app.js,index.html,styles.css} to the 4421944 design, embedded into the current hardened data flow rather than overwriting it: sessionSwitching, applyRuntimeEvent, prompt admission tokens, and resetLiveState are preserved.
• Ported the rendering layer: thinking rows with live timers (timed in applyRuntimeEvent, reset in resetLiveState), tool lines with icons/argument summaries and right-edge outcome glyphs (✓/✗/running dot), 4+ run grouping, family cards for subagent/workflow calls, per-turn copy/time message actions, turn rail, and composer activity chips adapted to the current { items, omitted } capability projections.
• Restored the settings dialog with browser-local persistence (openpi.language, openpi.web.theme) matching the original behavior; canonical setup-config integration remains future work tracked by feat(web): add persistent theme/language preferences and accessibility/mobile hardening #350.
• Restored favicon.svg and its static route in web/host/web-host.ts.
• Tests: converted d1118f8-era doesNotMatch lock-in assertions back to positive assertions for the revived UI surface; fixtures aligned to current protocol types.

Validation

• node --check web/ui/app.js — OK
• bun run check — config contract, biome format/lint, tsc all pass
• bun run test — 1236 tests, 1235 pass / 0 fail / 1 skip
• Not run: real-browser manual audit; visual fidelity targets the 4421944 snapshot.

Impact

• User-visible behavior: settings dialog (language/theme), landing logo animation + click replay, rich transcript rendering (thinking timers, tool status, activity cards), turn rail, activity bar, favicon, and grid backdrop all return.
• Model-visible context/tools: none.
• Runtime/lifecycle: none, except web-host.ts serving /favicon.svg from the existing static allowlist.
• Persisted config/data: two browser-local keys (openpi.language, openpi.web.theme); no canonical configuration changes.
• Compatibility/risk: low — no protocol or backend logic changes.

Refs #76. Relates to #345, #346, #350 (partial — restores the baseline these issues extend).

[tt-a1i]

我在 exact head 4724e5c 上完成了代码、完整测试和 Chromium 实机审查。bun run check 通过；bun run test 为 1235 pass / 0 fail / 1 skip。桌面布局、主题切换和设置弹窗正常，但当前有两个合并阻塞项：窄屏主界面会被整体模糊；另外本 PR 已经新增了持久化的主题/语言选择，却把仓库规定的统一配置契约留给 #350。建议本 PR 保持恢复 UI 基线的小范围：修掉窄屏 blur，并暂时移除设置持久化/入口（或在本 PR 完成统一配置契约）。修复后我可以立即复审。

[tt-a1i]

维护者已明确当前没有手机端范围，因此我撤回窄屏模糊作为本次合并阻塞项；该问题可以留到后续移动端支持时处理。当前只剩主题/语言持久化形成第二配置源这一项仓库配置契约问题。PR head 仍为 4724e5c，尚未包含对应修复。

[somewan820]

两个阻塞项均已在 1b340f2 处理（详见两条行内回复）：P1 窄屏整页模糊已修复（≤760px 侧栏改不透明、禁用 backdrop-filter，并补了窄屏回归断言）；P2 设置入口与主题/语言持久化已整体撤下，留给 #350 按统一配置契约实现。bun run check 与 bun run test（1235 pass / 0 fail / 1 skip）均通过，可以再次 review。