feat(ui): add linear Alice shell pass

[2233admin]

What

The "linear Alice shell" pass — reshapes the app shell toward a quiet, high-density agent workspace with Linear-grade navigation discipline:

• EmptyEditor home at / — a calm empty canvas centered on Ask Alice: mostly dark, low-contrast borders, large empty working area, one obvious object in the center. No marketing hero, no card pile, no loud gradients.
• ActivityBar / Sidebar / TabHost / TabStrip refined for density and stable row rhythm — tool-like, quiet hover/active states, compact labels.
• UrlAdopter two-way URL↔view sync tightened; coexists with the 0.41 source-aware search additions (useSearchParams + the shell's useLayoutEffect).
• i18n strings (en / ja / zh / zh-Hant) for the new shell surfaces.

Design target: a serious agent workspace at 2 a.m. Linear is the calibration ruler, not the skin — density, surface hierarchy, and restrained motion are borrowed; brand color / icons / issue-project IA are not.

Screenshots

Home — Ask Alice centered, empty canvas

Shell — tab density + chrome

Why

Foundation for the keyboard-first navigation spine — the Cmd/Ctrl+K command palette and g <letter> direct-jump nav land on top of this. Establishes the visual grammar (density, surface hierarchy, quiet interaction states) that later view work builds on.

Scope

UI-only, additive. 14 files, +231 / −85.

Area	Files
Shell components	ActivityBar, Sidebar, TabHost, TabStrip, EmptyEditor, App
URL / view bridge	tabs/UrlAdopter.tsx
i18n	i18n/locales/{en,ja,zh,zh-Hant}.ts
misc	index.css, vite-env.d.ts, package.json

Verification

• Rebased onto current master (0.41.0-beta.1). One trivial import-line merge in UrlAdopter.tsx (union of useSearchParams + useLayoutEffect); all other files auto-merged clean.
• tsc -b green.
• gitleaks clean (no secrets).
• ui vitest suite: a pre-existing environment failure exists on master (jsdom localStorage under vitest 4.x) — this PR touches none of the affected specs (tabs/store, watchlist-store) and introduces no new failures.

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