Story-2030: Route the placeholder modes — Inbox, Strategy, Chronology, Proof

The four not-yet-built Work Modes — Inbox, Strategy, Chronology, Proof — become real, deep-linkable routes rendered via <Link>, and the last of the in-shell activeMode client state machine is removed. This continues EPIC-2018 (route-per-mode) on top of the fat layout (S1, Story-2019) and the Brief route (S2, Story-2029). The bodies are still the honest "lands in a later release" placeholder — navigable shells, not fake-data screens.

What moved

• Four new thin routes — matters/[matterId]/{inbox,strategy,chronology,proof}/page.tsx. Each renders only its body under the existing fat layout.tsx (which already validates locale/matterId and hosts the shared chrome — Ribbon + Worknav + Rail), so the pages carry no fetch and no chrome. Each exports an agentSurface (matter.inbox / matter.strategy / matter.chronology / matter.proof) with a brain-facing description and a presence key added to both en.json and hi.json.
• Shared placeholder body — the dashed tabPlaceholder block that used to live inside LiveMatterShell's activeMode branch is now the server component matter-detail/matter-mode-placeholder.tsx, reused by all four routes. Same markup, same i18n keys (MatterDetail.tabPlaceholder / tabs.* / inbox), which already existed in both locales.
• Worknav tabs are now all links — the four modes were added to SURFACE_KEY_BY_MODE / MODE_BY_SURFACE_KEY in matter-worknav.tsx, so every Worknav tab now renders as a real <a href> to buildSurfacePath (right-clickable / open-in-new-tab), with the active tab derived from the pathname. The map is no longer Partial — all eight modes are routed.

The activeMode state machine is gone

LiveMatterShell lost its activeMode / setActiveMode / onModeChange machinery (and the useState / useCallback / useRouter imports that drove it). Mode switching is now pure routing — the Worknav links navigate, the fat layout keeps the chrome put, and the matched route renders its body. The file is now unused chrome slated for deletion in S7 (#2034); this story only strips the machinery per its scope, and leaves LiveMatterDetailPage untouched.

Deliberately deferred (not this story)

• Real Inbox / Strategy / Chronology / Proof surfaces — the bodies stay placeholders until their own stories land.
• Re-homing Record and splitting Filings / Hearings into subpages (S4–S6), and deleting LiveMatterShell / LiveMatterDetailPage / MatterSubnav and the orphan review/ dir (S7, #2034).

Verification

pnpm gen:routes + pnpm check:routes (34 surfaces, fresh), pnpm lint (0 errors), pnpm test (1001 passed), and pnpm build are green. The build output lists all four new routes: /[locale]/matters/[matterId]/{inbox,strategy,chronology,proof}. No codegen / contract change (placeholder bodies have no data path).