Add sidebar lifecycle filters and drafts

[brsbl]

What was wrong

The built-in sidebar thread list represented only active server threads. Client-local new-thread drafts disappeared from navigation after leaving a composer, archived threads were available only through Settings, and the client had no count-only archived read for showing lifecycle totals without fetching archived rows.

What changed

• Adds a session-local Show checkbox group for Active, Drafts, and Archived, with live counts, Active as the launch default, and reset-on-launch behavior.
• Renders client-local draft rows in the built-in list's top cluster, re-enters a draft only from its row, preserves its submit destination, removes empty/submitted drafts, and shows Saved to Drafts once when navigating away from a non-empty composer.
• Adds a contiguous trailing archived group with incremental paging, archived-row navigation and unarchive actions, and an empty-filter state.
• Adds the smallest count-only server/SDK read for active or archived threads, so the Show menu can display archived totals without loading archived rows.
• Keeps drafts and preferences entirely client-local, leaves plugin-rendered composers and experimental_threadList replacement rendering unchanged, and deliberately defers search to its future command-palette layer.
• Does not change the host-daemon wire protocol or add server-side draft state.

Before — parent PR #2404 (e503e4d35)

The built-in list has no lifecycle controls and exposes neither local drafts nor archived threads.

After — this PR (b971bd795)

The same fixture and 1440×900 viewport show the Draft top cluster, Active rows, trailing Archived group, and the Show menu with all three live counts.

How you verified

• Phase 2 focused regressions: app 73, server routes 80, database 53, SDK 51, and server-contract 35 tests passed (292 total).
• pnpm exec turbo run typecheck --force: 76/76 tasks passed under Node 22.22.1.
• pnpm exec turbo run lint --force: 2/2 tasks passed; only existing warnings remained.
• pnpm exec turbo run test --force: 69/75 tasks passed. The changed app, server-route, database, SDK, and contract coverage passed; the remaining failures were outside this diff in provider recorded-replay parity and plugin-build /tmp path normalization, plus two server timeout cases that passed together on immediate direct rerun (33/33).
• Chrome for Testing 151.0.7922.71 and Safari 26.5.2: passed first paint, hard reload, client navigation, five menu cycles, lifecycle reset, draft toast, archived unarchive/empty state, light/dark/system themes, and exact mobile/small/desktop/very-wide plus breakpoint-adjacent widths with no overflow or console errors.

BB-Thread-ID: thr_2e2gbjx943

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