refactor(tui): store-driven turn state + slash registry + module split

Hoist turn state from a 286-line hook into $turnState atom + turnController
singleton. createGatewayEventHandler becomes a typed dispatch over the
controller; its ctx shrinks from 30 fields to 5. Event-handler refs and 16
threaded actions are gone.

Fold three createSlash*Handler factories into a data-driven SlashCommand[]
registry under slash/commands/{core,session,ops}.ts. Aliases are data;
findSlashCommand does name+alias lookup. Shared guarded/guardedErr combinator
in slash/guarded.ts.

Split constants.ts + app/helpers.ts into config/ (timing/limits/env),
content/ (faces/placeholders/hotkeys/verbs/charms/fortunes), domain/ (roles/
details/messages/paths/slash/viewport/usage), protocol/ (interpolation/paste).

Type every RPC response in gatewayTypes.ts (26 new interfaces); drop all
`(r: any)` across slash + main app.

Shrink useMainApp from 1216 -> 646 lines by extracting useSessionLifecycle,
useSubmission, useConfigSync. Add <Fg> themed primitive and strip ~50
`as any` color casts.

Tests: 50 passing. Build + type-check clean.

ui-tui/src/app/useComposerState.ts

@@ -7,12 +7,12 @@ import { useStore } from '@nanostores/react'
 import { useCallback, useMemo, useState } from 'react'
 
 import type { PasteEvent } from '../components/textInput.js'
+import { LARGE_PASTE } from '../config/limits.js'
 import { useCompletion } from '../hooks/useCompletion.js'
 import { useInputHistory } from '../hooks/useInputHistory.js'
 import { useQueue } from '../hooks/useQueue.js'
 import { pasteTokenLabel, stripTrailingPasteNewlines } from '../lib/text.js'
 
-import { LARGE_PASTE } from './constants.js'
 import type { PasteSnippet, UseComposerStateOptions, UseComposerStateResult } from './interfaces.js'
 import { $isBlocked } from './overlayStore.js'