Add a file explorer with prompt tagging and improve transcript readability

[DevMando]

Summary

Adds a built-in file explorer to each chat tab, new point-and-click ways to reference project files in a prompt ("tagging"), and readability improvements to the chat transcript — including making saved transcript files fully interactive.

What's new

File explorer (per chat tab)

• A new toolbar icon (next to the change-folder icon) opens a file tree of the tab's project folder, docked on the right side of the chat. The chat resizes to make room, so no conversation text is ever hidden behind it.
• The panel can be resized by dragging its left edge, and remembers the chosen width for the session.
• Folders load their contents only when expanded, so even very large projects open instantly.
• Double-clicking a file opens it in its default Windows app; a refresh button re-reads the tree.

Tagging files into the prompt

• Every row in the tree has an "@" button — clicking it inserts that file or folder into the prompt exactly as if the user had typed it with the existing @ autocomplete. Hovering the button shows the exact reference it will insert.
• Files and folders can also be dragged into the prompt — from the explorer tree or straight from Windows Explorer.
• Dragging anything over the chat area shows a large "Drop to tag in prompt" target over the transcript, so drops don't have to hit the small input box.
• Items outside the tab's project folder are skipped with a clear warning, since the assistant can only read files inside the project.

Transcript readability

• Very long pasted prompts now collapse to about 8 lines with a "Show more (N more lines)" button. The full text is still sent to the assistant and can be expanded at any time.
• Prompt styling was tuned so large pastes no longer visually overpower the assistant's responses.
• Saved transcript HTML files are now fully interactive: the Show more, expand/collapse, and copy buttons all work when the file is opened in a regular browser (previously these buttons did nothing in saved files).

Why it matters

Referencing project files is core to how people use the app, and previously required typing @ paths by hand. This release makes it point-and-click and drag-and-drop, and cleans up the transcript for long working sessions and for sharing exported conversations.

Scope and risk

• All changes are in the desktop UI layer (the chat tab control and the transcript renderer). No changes to assistant behavior, file-access rules, or stored data.
• The new tagging UI produces the same @references as the existing autocomplete, so downstream handling is unchanged.
• The existing terminal, snapshots, and @ autocomplete are untouched (the terminal's resize control was extended, not modified, to support the new panel).

How to verify

1. Open a chat tab and click the new explorer icon: the file tree appears and the chat shrinks to fit; drag the panel's left edge to resize it.
2. Click the "@" on any row, and drag a file from the tree and from Windows Explorer onto the chat: an @reference lands in the prompt each time.
3. Double-click a file in the tree: it opens in its default app.
4. Paste 20+ lines into the prompt and send it: the message collapses with "Show more". Save the transcript (save button) and confirm Show more and Copy work when the saved file is opened in a browser.