feat(web): polish landing, model picker, and sidebar presentation

[tt-a1i]

Problem

The web workbench landing and model picker looked unfinished: the giant landing wordmark was clipped by the model-picker popup when it opened (no explicit z-index on the selector layer), the no-workspace composer was a bare dashed box floating in dead space with no clear call to action, the model list was a flat wall of names with heading-only provider groups and no persistent selected state, and the sidebar lacked visual hierarchy.

Value

First-run and empty-session screens are the product's front door. A composed landing, a legible model picker, and a calmer sidebar make the workbench feel deliberate without changing any behavior.

Approach

Purely presentational, scoped to web/ui:

• Landing: scale the animated wordmark to 0.82, add a bilingual product tagline (landingTagline), center the workspace picker, and give the astryx selector popup an explicit z-index: 60 so it always stacks above the hero.
• Dormant composer (no workspace): hide the read-only textarea and render a folder icon + "choose a workspace" CTA row instead; subtle tinted background and hover feedback; overlay button semantics unchanged.
• Model picker: bordered/elevated popup surface, sticky uppercase provider group headings, two-line options (name + monospaced model id), persistent aria-selected highlight with accent check, and a separated, de-emphasized "model settings" footer item.
• Sidebar: card-style bordered new-session button, small-caps section label, subtler session timestamps, footer divider. All values derive from existing theme variables, so dark/pine/mist/rose themes follow automatically.

Validation

• bun run check (contracts + check:web + biome format/lint + tsc) — pass
• bun run test — 1158 tests, 82 files, pass
• bunx playwright test tests/web/workbench-polish.e2e.ts — 8 pass
• Manual browser verification against a live workbench: landing in dark and light themes, model picker (thinking view + list view) in both themes, and the no-workspace dormant state via an isolated --no-workspace instance. Theme preference restored afterwards.

Impact

• User-visible behavior: landing/model-picker/sidebar visuals only; no interaction, command, or lifecycle changes.
• Model-visible context/tools: none.
• Runtime/lifecycle: none.
• Persisted config/data: none (one new i18n key in en + zh).
• Compatibility/risk: low; CSS selector additions are additive, markup changes are limited to presentation-only nodes (tagline, dormant CTA, option sub-label).