fix(desktop): stabilize composer during session switches and resizing

[colaforniaw]

Summary

Switching Sessions could briefly show controls from the previous Session, while narrow Composer widths could compress the footer into the send button. An empty Composer could also show the branch icon through the translucent disabled Send button.

This change keeps the Composer footer on one row with a 520px minimum conversation width, preserves fixed space for Send, prevents stale session branch state from painting during a switch, and gives the Send/Stop slot an opaque backing while keeping the existing disabled appearance.

The Workbar remains under user control; narrow widths do not automatically move or collapse it.

Before / After

Scenario	Before	After
Narrow window: Composer footer in desktop	placeholder	after-responsive.mp4
Narrow window: Composer footer in workhub	placeholder	workhub-responsive-after.mp4
Session switch: stale controls and usage flicker	placeholder	switch-after.mp4

Verification

• Manual visual verification confirmed the Composer remains single-line, the Send button no longer reveals the branch icon, and Session switching no longer shows stale branch state.
• git diff --check passed.
• npm --workspace @maka/desktop run check:architecture -- --base d0dd3edf202f120c5ea950825160dfd02b426145 --strict-base passed, including 112 architecture checker tests.
• Final typecheck and focused tests were not rerun after the last visual fixes; they should be run before merge.

AI use

• No generative tool made a substantive contribution
• Generative tooling made a substantive contribution

Tool(s) and scope: Codex assisted with the Composer behavior, layout, usage state, and regression coverage. The commit carries Generated-by: Codex.

Checklist

• Tests cover the change and fail without it
• Lint, format, typecheck and the affected suites pass locally

Does this PR entail a change in behavior?

• Yes — described under Summary above
• No