Size HostUI from the window, not 300/360/440 locks

[jsoneaday]

Summary

• Reply pane width, list padding, bubble gutters, and composer insets follow the window and pane instead of the old 300/360/440 point locks.
• Message bubbles still use the pane-fraction cap (72%, 520 pt ceiling).
• Previews cover narrow / default / wide for channel, thread, and composer.

Closes #18

Test plan

• Open a connector inbox and resize from a narrow window to a wide one; the reply pane should grow with the window, not stay ~360 pt.
• Open a thread; bubbles should still take about three-quarters of that pane, not the full window.
• Check channel, thread, and composer at a small, default, and large window.
• Confirm HostUI still looks like the current messaging chrome.

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