[matthewrball]

Why

The task header's height and separate background band add visual space above the canvas. This draft isolates the toolbar proposal from #1347 so its appearance can be reviewed separately.

What this PR adds

• Reduce the shared task-header and collapsed-rail clearance from 40px to 36px, keeping existing controls centered within it.
• Use the canvas background for the header and main-pane wrapper, and remove the canvas's upper-left rounding so the task area forms one continuous surface.

This is the exact final toolbar patch extracted from #1347: two files, with no sidebar, Settings, Search, or folder-selector behavior changes. It targets master independently; the sidebar/Settings work stays in #1347 and draft folder selection stays in #1353. Kept as a draft for design discussion.

Preview

Native design reference from the combined prototype. The sidebar styling and title-bar Search shown here belong to #1347 and are not included in this draft's diff.

Verification

• At f800e39b, formatting and git diff --check passed; independent source review found no issues.
• The extracted patch matches the original toolbar change exactly. Recombining it with the narrowed Agent: polish sidebar navigation and settings menu #1347 produces the same Git tree as the previously approved changes on current master.
• After removing this patch, Agent: polish sidebar navigation and settings menu #1347's complete app suite passed: 1,145 passed, 0 failed, 2 ignored.
• The standalone draft's app-test build stopped before tests ran because the local disk ran out of space. Its just ci attempt also stopped on temporary-file allocation errors. In the Agent: polish sidebar navigation and settings menu #1347 checkout, just ci reached the previously observed Rust 1.96 Clippy errors in unchanged code. No checks or assertions were disabled.

Local checks use the previously authorized Rust 1.96.0 / Xcode 27.1 environment. Pinned-toolchain CI and platform checks require maintainer approval. The combined native design was previously inspected; no new app instance was launched for this extraction.

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