chore(web): extract App.tsx's leaf hooks (#2128)

Phase 1 of #2126. Six hooks move out of `clients/web/src/App.tsx`, taking
its component body from 4,891 to 4,267 lines with no behavior change:

- useThemeToggle       — the light/dark toggle
- useTabUiState        — the nine per-screen `ui` objects, the active tab,
                         the pinned-Protocol ids, and the session reset
- useInspectorStores   — the twelve state managers as one create/destroy
                         lifecycle, their `core/react` hooks, and the
                         pagination-mode selection
- useExportActions     — the eleven Clear / Export / Replay handlers
- useProgressToasts    — `notifications/progress` toasts
- useTaskToasts        — task-status toasts, the cancel confirmation, the
                         taskId -> progress map, and the in-flight
                         tool-call taskId ref

Holding the stores in one slot collapses `setupClientForServer`'s and
`onConfirmRemove`'s dependency arrays from twelve entries to two stable
callbacks; `destroyStores` reads the live set through a ref mirror rather
than the state it also writes.

All six hooks are at 100% on all four coverage dimensions, no `v8 ignore`
needed. Also refreshes the now-stale "~4.5k-line" description of App.tsx
in AGENTS.md and the coverage `include` comment.

Signed-off-by: cliffhall <cliff@futurescale.com>

AGENTS.md

@@ -1058,7 +1058,7 @@ The top-level **`src/types/`** is a sibling of both and is not the place for new
 
 Cross-directory imports point **one way, `lib → utils`** (infra depends on pure helpers, never the reverse). Keep it that way: if a `utils/` module needs a type currently exported from a `lib/` module, declare the type in `utils/` and re-export it from `lib/` (as `pendingReauth` owns `OAuthResumeAuthKind` and `oauthResume` re-exports it), rather than importing "up" from `utils` into `lib`.
 
-Nothing **enforces** the boundary: no path alias keys off it, and the coverage `include` in `clients/web/vite.config.ts` lists **both** `src/lib/**` and `src/utils/**`, so a move between them is coverage-neutral (this is why the refactor was gate-safe). It's a human-legible signal at import time, valuable in a codebase this test-heavy (the ≥90% per-file gate). Note that `include` is a **whitelist** — it names `components`/`hooks`/`theme`/`lib`/`utils`/`server` (plus the `core/*` runtime; `hooks` and `theme` were added in #1787), so a module placed **outside** those directories (`types/`, `App.tsx`, or a brand-new grab-bag) falls out of the ≥90 gate entirely, silently. The **deliberate, documented** top-level-file exceptions are `src/App.tsx` — a ~4.5k-line composition root at ~42% branch coverage (gating it is a dedicated testing/decomposition effort, not a whitelist tweak) — and the `src/main.tsx` / `src/index.ts` bootstraps (browser `createRoot` render and the bin `runWeb` re-export, the analog of `clients/cli`'s excluded `src/index.ts`). All three are called out in a comment on the `include` array itself rather than left silent. When adding a module, place it by the rule and keep it inside a gated directory; when it genuinely mixes both (e.g. `downloadFile` bundles DOM-side-effect helpers with a couple of pure ones), keep it whole on its dominant side (`lib`) rather than splitting hairs.
+Nothing **enforces** the boundary: no path alias keys off it, and the coverage `include` in `clients/web/vite.config.ts` lists **both** `src/lib/**` and `src/utils/**`, so a move between them is coverage-neutral (this is why the refactor was gate-safe). It's a human-legible signal at import time, valuable in a codebase this test-heavy (the ≥90% per-file gate). Note that `include` is a **whitelist** — it names `components`/`hooks`/`theme`/`lib`/`utils`/`server` (plus the `core/*` runtime; `hooks` and `theme` were added in #1787), so a module placed **outside** those directories (`types/`, `App.tsx`, or a brand-new grab-bag) falls out of the ≥90 gate entirely, silently. The **deliberate, documented** top-level-file exceptions are `src/App.tsx` — a ~4.3k-line composition root at ~42% branch coverage (gating it is a dedicated testing/decomposition effort, not a whitelist tweak) — and the `src/main.tsx` / `src/index.ts` bootstraps (browser `createRoot` render and the bin `runWeb` re-export, the analog of `clients/cli`'s excluded `src/index.ts`). All three are called out in a comment on the `include` array itself rather than left silent. When adding a module, place it by the rule and keep it inside a gated directory; when it genuinely mixes both (e.g. `downloadFile` bundles DOM-side-effect helpers with a couple of pure ones), keep it whole on its dominant side (`lib`) rather than splitting hairs.
 
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