fix(desktop): preserve empty composer caret height

[sunrioa]

Summary

Fix the clipped insertion caret in an empty composer after switching between WorkHub and sidebar conversations.

Keep the scrolling editable at least one line tall, including its vertical padding. Share the padding value with compact WorkHub so both layouts retain enough space for the caret.

Preserve the existing interaction gate, editor DOM, draft restoration and multiline scrolling. Add regression coverage to the existing EmptyHome and ProgressModelPicker stories.

Fixes #5264

Verification

• Regression checks fail without the fix on baseline 6bff26b42: EmptyHome collapses to 8px instead of at least 30px; compact WorkHub measures 22px instead of at least 34px. All three focused browser cases pass with the fix.
• Covered repeated inert/hide/show transitions, draft restoration, empty disabled/enabled states, multiline scrolling and send-clear.
• Storybook typecheck and build passed. Full Storybook smoke passed: 359 stories / 388 theme renders.
• With Node 24.19.0 and npm 11.19.0, lint, format:check, build, typecheck and Knip for apps/desktop and packages/ui passed.
• Additional broad validation exercised all 11 workspaces. Desktop (2593/2593), UI, Core, Storage, MCP, Computer Use, Website and Eval passed. Runtime and CLI passed complete workspace reruns after failing in the concurrent run. Runtime Host still had three process-lifecycle failures in its complete isolated rerun; the three involved files subsequently passed separately (95/95). This is not a clean full-suite result. No assertions or timeouts were relaxed, and this patch does not change those packages.
• Not run on this branch: Windows native navigation, real OS IME candidate-window validation, or Electron E2E. Browser geometry tests do not establish a Chromium engine root cause or verify native caret pixels.

Before/after layout evidence (the browser assertions above verify the editable height; these screenshots are not native caret-pixel assertions):

Before:

After:

AI use

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

Tool(s) and scope: OpenAI Codex — implementation, regression tests, and validation; reviewed by the author.

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