UI Theming System & Chat Experience Modernization

[DevMando]

What this PR does

This PR transforms MandoCode Desktop from a functional prototype into a polished,
personalizable app. It delivers three things: a full theming system, a
reorganized Settings experience, and a modernized chat transcript with the
affordances users expect from contemporary AI chat apps (VS Code, Claude, Cursor).

No changes to the AI engine — the CLI submodule stays pinned at v0.13.0. Everything
here is desktop UI.

Why

• The app's pages looked inconsistent (chat was themed; Settings/MCP followed the
  OS instead), which read as unfinished.
• Settings was a single long scroll with unexplained, technical toggles.
• The chat transcript was plain text: no code coloring, no copy actions, and tool
  activity drowned out the conversation on long agent runs.
• Several approval-flow papercuts interrupted users at the wrong times.

What's new

🎨 Appearance

• 13 selectable themes — including Visual Studio Dark (new default), Dracula,
  Monokai, One Dark Pro, Tokyo Night, and 4 light themes. Every surface recolors
  live, including the chat, with no restart.
• Window opacity slider (30–100%) for a translucent always-on-screen setup.
• Choices persist per-machine and never touch the config file shared with the CLI.

⚙️ Settings

• Reorganized into 6 tabs: Appearance, Connection, Generation, Behavior,
  Limits, Integrations.
• Every setting now has a plain-English tooltip explaining how it changes the
  AI's behavior.
• Fixed: the Model field sometimes showed placeholder text instead of the active
  model; the Tavily API key is now masked with an explicit View/Hide toggle.

✅ Approvals (the "review before the AI acts" flow)

• Approval dialogs now appear only in the chat page — they no longer interrupt
  work in Settings/MCP. A clickable toast notification (plus a gold nav-icon
  indicator) appears if an approval arrives while you're elsewhere; clicking it
  jumps straight to the waiting dialog.
• "Provide new instructions" opens a full-width editor with room to write, and
  Cancel returns to the original approval instead of abandoning it (misclick
  recovery).
• Option buttons now carry icons (approve ✓ / deny ✗ / edit ✎ / stop ⏹).

💬 Chat transcript

• Syntax highlighting for code in ~40 languages, colored to match the active
  theme (bundled locally — works fully offline).
• Copy buttons on hover for any code block or whole reply.
• Tool-activity grouping: consecutive operations collapse into one expandable
  "⚙ N operations" row once the AI replies — long agent runs stay readable.
• Ctrl+F in-chat search with match count and navigation.
• "↓ Latest" pill when scrolled away from the newest message.
• Clickable file paths — any file the AI touches opens in your default app.
• Save transcript button — exports the conversation as a standalone HTML file,
  colors intact (useful for sharing agent runs).
• Fixed a message-ordering bug where multi-step responses posted out of order
  relative to approval prompts.

📄 Compliance

• Added THIRD-PARTY-NOTICES.txt covering all bundled/compiled open-source
  components (a pre-Store-listing requirement).