feat: Add streaming chat + scroll persistence; improve markdown & links

Backend
- /chat: support streaming via StreamingResponse; save full reply after stream ends. Non-stream path unchanged.
- ChatRequest: add stream flag (default false).
- GenerateTitleRequest: add model and use it instead of hardcoded llama3.
- ollama_client.chat_stream(): new async generator parsing Ollama streaming JSON (both formats).
- Remove response_model from /chat to allow streaming; non-stream still returns { reply }.

Electron
- Open external links in system browser (setWindowOpenHandler, shell.openExternal).
- New IPC: update-settings, open-external-link.
- Set minimum window size; preload exposes updateSettings and openExternalLink.

Frontend (React)
- Streaming UI with live chunking; sticky-bottom only when user at bottom.
- Per-session scroll persistence and robust restore.
- New message tip to jump to latest reply when scrolled up.
- Disable Send while sending; spinner.
- General Settings: stream output toggle; propagate model/stream changes.
- Apply color scheme at boot; extract colorSchemes helper.
- Sidebar UX tweaks and unread badges.

Markdown/rendering
- Code blocks: language title bar and wrapper.
- Tables: GitHub-style parsing, per-cell borders, rounded wrapper, spacing, alignment.
- Headings: remove blank line after h1-h4.
- <hr>: handle after tables; strip following whitespace.
- Links: target=_blank with icon and URL tooltip.

Styles
- Add styles for code/table wrappers, new-message tip, toggle, spinner; hover/active vars; narrower sidebar.

API notes / breaking changes
- /chat accepts stream=true and returns text/plain streamed chunks.
- generate-title now requires a model.
- Non-stream /chat response shape unchanged.

src/InterfaceSettings.jsx

@@ -1,52 +1,8 @@
 import React, { useState, useEffect } from 'react';
+import { colorSchemes, applyColorScheme } from './colorSchemes';
 
 const COLOR_SCHEME_KEY = 'colorScheme';
 
-const colorSchemes = {
-  'Default': {
-    '--bg': '#0b1020',
-    '--panel': '#141b34',
-    '--text': '#e6e8ef',
-    '--muted': '#9aa3b2',
-    '--accent': '#6ea8fe',
-    '--border': '#24304f',
-    '--input-bg': '#0e1530',
-    '--user-msg-bg': '#111933',
-    '--assistant-msg-bg': '#101927',
-  },
-  'Grayscale': {
-    '--bg': '#1a1a1a',
-    '--panel': '#2a2a2a',
-    '--text': '#f0f0f0',
-    '--muted': '#aaaaaa',
-    '--accent': '#888888',
-    '--border': '#4a4a4a',
-    '--input-bg': '#202020',
-    '--user-msg-bg': '#333333',
-    '--assistant-msg-bg': '#252525',
-  },
-  'Rose': {
-    '--bg': '#200a10',
-    '--panel': '#301a20',
-    '--text': '#ffe0e0',
-    '--muted': '#a09090',
-    '--accent': '#E91E63',
-    '--border': '#402025',
-    '--input-bg': '#2a1015',
-    '--user-msg-bg': '#331119',
-    '--assistant-msg-bg': '#271019',
-  },
-};
-
-function applyColorScheme(schemeName) {
-  const scheme = colorSchemes[schemeName];
-  if (scheme) {
-    for (const [key, value] of Object.entries(scheme)) {
-      document.documentElement.style.setProperty(key, value);
-    }
-  }
-}
-
 export default function InterfaceSettings() {
   const [selectedColorScheme, setSelectedColorScheme] = useState('Default');