fix(desktop): prevent chat-pane white-screen on narrow windows

[arccat-114]

Problem

When the Reasonix desktop window is snapped to the left/right half of the screen (Windows Snap Assist), the right-side chat-pane area becomes completely white/blank.

Reproduction:

1. Open Reasonix desktop client
2. Drag window to left/right screen edge to trigger half-screen snap
3. Observe the right-side chat-pane area

Actual result: chat-pane area is completely white. Left sidebar, top tab bar, and bottom status bar remain normal.

Expected result: chat-pane should display chat content normally.

Screenshots: (to be added — showing white chat-pane at half-screen width, and content disappearing as window narrows)

Root Cause

1. CSS Grid uses minmax(0, 1fr) for the chat-pane column, allowing it to be compressed to 0 width
2. Window MinWidth was set to 760px, preventing the window from being resized below half-screen
3. At half-screen width (~960px), sidebar takes ~264px, leaving ~696px for chat-pane — but internal content needs more space, causing a rendering failure (white screen)

Fix

1. CSS Grid — enforce chat-pane minimum width (desktop/frontend/src/styles.css)

• Lowered --chat-min-width from 760px to 320px
• Changed all minmax(0, 1fr) to minmax(var(--chat-min-width), 1fr) in grid-template-columns, ensuring the chat column is never compressed below 320px

2. Auto-collapse sidebar on narrow viewports (desktop/frontend/src/App.tsx)

• Added a useEffect that monitors viewportWidth
• When viewportWidth < sidebarWidth + CHAT_MIN_WIDTH (664px), automatically collapses the sidebar
• Ensures chat-pane always has at least 400px of width

3. Lower window minimum width (desktop/main.go)

• Reduced MinWidth from 760 to 400
• Allows the window to be resized to smaller dimensions

Testing

• Window can now be resized down to 400px minimum width
• Sidebar auto-collapses when viewport drops below ~664px
• Chat-pane always maintains at least 320px width — no white screen
• Half-screen snap works correctly with chat-pane visible