feat: add auto-chunking limit slider to settings

Persisted setting (default 800 chars) controls how long text is split
before generation.  Lower values improve quality for long outputs by
keeping each chunk well within the model's context window.

- Slider in Server Connection settings (100–2000 chars, step 50)
- Stored in localStorage via Zustand persist
- Passed as max_chunk_chars on every generation request
- Frontend text limit raised to 50,000 to match backend

app/src/stores/serverStore.ts

@@ -13,6 +13,9 @@ interface ServerStore {
 
   keepServerRunningOnClose: boolean;
   setKeepServerRunningOnClose: (keepRunning: boolean) => void;
+
+  maxChunkChars: number;
+  setMaxChunkChars: (value: number) => void;
 }
 
 export const useServerStore = create<ServerStore>()(
@@ -29,6 +32,9 @@ export const useServerStore = create<ServerStore>()(
 
       keepServerRunningOnClose: false,
       setKeepServerRunningOnClose: (keepRunning) => set({ keepServerRunningOnClose: keepRunning }),
+
+      maxChunkChars: 800,
+      setMaxChunkChars: (value) => set({ maxChunkChars: value }),
     }),
     {
       name: 'voicebox-server',