refactor(chat): split monolithic chat interface into typed modules and hooks

Replace the legacy monolithic ChatInterface.jsx implementation with a modular TypeScript architecture centered around a small orchestration component (ChatInterface.tsx).

Core architecture changes:
- Remove src/components/ChatInterface.jsx and add src/components/ChatInterface.tsx as a thin coordinator that wires provider state, session state, realtime WebSocket handlers, and composer behavior via dedicated hooks.
- Update src/components/MainContent.tsx to use typed ChatInterface directly (remove AnyChatInterface cast).

State ownership and hook extraction:
- Add src/hooks/chat/useChatProviderState.ts to centralize provider/model/permission-mode state, provider/session synchronization, cursor model bootstrap from backend config, and pending permission request scoping.
- Add src/hooks/chat/useChatSessionState.ts to own chat/session lifecycle state: session loading, cursor/claude/codex history loading, pagination, scroll restoration, visible-window slicing, token budget loading, persisted chat hydration, and processing-state restoration.
- Add src/hooks/chat/useChatRealtimeHandlers.ts to isolate WebSocket event processing for Claude/Cursor/Codex, including session filtering, streaming chunk buffering, session-created/pending-session transitions, permission request queueing/cancellation, completion/error handling, and session status updates.
- Add src/hooks/chat/useChatComposerState.ts to own composer-local state and interactions: input/draft persistence, textarea sizing and keyboard behavior, slash command execution, file mentions, image attachment/drop/paste workflow, submit/abort flows, permission decision responses, and transcript insertion.

UI modularization under src/components/chat:
- Add view/ChatMessagesPane.tsx for message list rendering, loading/empty states, pagination affordances, and thinking indicator.
- Add view/ChatComposer.tsx for composer shell layout and input area composition.
- Add view/ChatInputControls.tsx for mode toggles, token display, command launcher, clear-input, and scroll-to-bottom controls.
- Add view/PermissionRequestsBanner.tsx for explicit tool-permission review actions (allow once / allow & remember / deny).
- Add view/ProviderSelectionEmptyState.tsx for provider and model selection UX plus task starter integration.
- Add messages/MessageComponent.tsx and markdown/Markdown.tsx to isolate message rendering concerns, markdown/code rendering, and rich tool-output presentation.
- Add input/ImageAttachment.tsx for attachment previews/removal/progress/error overlay rendering.

Shared chat typing and utilities:
- Add src/components/chat/types.ts with shared types for providers, permission mode, message/tool payloads, pending permission requests, and ChatInterfaceProps.
- Add src/components/chat/utils/chatFormatting.ts for html decoding, code fence normalization, regex escaping, math-safe unescaping, and usage-limit text formatting.
- Add src/components/chat/utils/chatPermissions.ts for permission rule derivation, suggestion generation, and grant flow.
- Add src/components/chat/utils/chatStorage.ts for resilient localStorage access, quota handling, and normalized Claude settings retrieval.
- Add src/components/chat/utils/messageTransforms.ts for session message normalization (Claude/Codex/Cursor) and cached diff computation utilities.

Command/file input ergonomics:
- Add src/hooks/chat/useSlashCommands.ts for slash command fetching, usage-based ranking, fuzzy filtering, keyboard navigation, and command history persistence.
- Add src/hooks/chat/useFileMentions.tsx for project file flattening, @mention suggestions, mention highlighting, and keyboard/file insertion behavior.

TypeScript support additions:
- Add src/types/react-syntax-highlighter.d.ts module declarations to type-check markdown code highlighting imports.

Behavioral intent:
- Preserve existing chat behavior and provider flows while improving readability, separation of concerns, and future refactorability.
- Move state closer to the components/hooks that own it, reducing cross-cutting concerns in the top-level chat component.

src/components/chat/input/ImageAttachment.tsx

@@ -0,0 +1,48 @@
+import { useEffect, useState } from 'react';
+
+interface ImageAttachmentProps {
+  file: File;
+  onRemove: () => void;
+  uploadProgress?: number;
+  error?: string;
+}
+
+const ImageAttachment = ({ file, onRemove, uploadProgress, error }: ImageAttachmentProps) => {
+  const [preview, setPreview] = useState<string | undefined>(undefined);
+  
+  useEffect(() => {
+    const url = URL.createObjectURL(file);
+    setPreview(url);
+    return () => URL.revokeObjectURL(url);
+  }, [file]);
+  
+  return (
+    <div className="relative group">
+      <img src={preview} alt={file.name} className="w-20 h-20 object-cover rounded" />
+      {uploadProgress !== undefined && uploadProgress < 100 && (
+        <div className="absolute inset-0 bg-black/50 flex items-center justify-center">
+          <div className="text-white text-xs">{uploadProgress}%</div>
+        </div>
+      )}
+      {error && (
+        <div className="absolute inset-0 bg-red-500/50 flex items-center justify-center">
+          <svg className="w-6 h-6 text-white" fill="none" stroke="currentColor" viewBox="0 0 24 24">
+            <path strokeLinecap="round" strokeLinejoin="round" strokeWidth={2} d="M6 18L18 6M6 6l12 12" />
+          </svg>
+        </div>
+      )}
+      <button
+        onClick={onRemove}
+        className="absolute -top-2 -right-2 bg-red-500 text-white rounded-full p-1 opacity-0 group-hover:opacity-100"
+      >
+        <svg className="w-3 h-3" fill="none" stroke="currentColor" viewBox="0 0 24 24">
+          <path strokeLinecap="round" strokeLinejoin="round" strokeWidth={2} d="M6 18L18 6M6 6l12 12" />
+        </svg>
+      </button>
+    </div>
+  );
+};
+
+export default ImageAttachment;
+
+