feat(image): convert HEIC paste uploads to JPEG

When a browser pastes an HEIC file without normalising it first, the
paste-image route now converts it to JPEG server-side via heic-convert
before writing to .claude-images/. Magic-byte validation confirms the
output is valid JPEG. Adds type declarations for the heic-convert package.

Co-authored-by: Saqeb Akhter <saqeb.akhter@gmail.com>

src/types/heic-convert.d.ts

@@ -0,0 +1,9 @@
+declare module 'heic-convert' {
+  export interface ConvertOptions {
+    buffer: Buffer;
+    format: 'JPEG' | 'PNG';
+    quality?: number;
+  }
+
+  export default function convert(options: ConvertOptions): Promise<Buffer | Uint8Array | ArrayBuffer>;
+}