Keep Editor header actions reachable in narrow panes

[callumalpass]

What changed

• reserve a fixed action cluster in note and type list headers
• constrain long collection titles and count summaries to the remaining pane width
• add a browser regression covering 304 px and minimum 240 px note panes, including opening the new-note composer

Why

A long collection name could contribute its intrinsic width to the flex row and push Hide notes sidebar and New note hundreds of pixels outside the pane. This was visible with the large mdbase-reader staging collection.

Impact

Header controls remain reachable at narrow desktop pane widths without changing the mobile pane model. The shared structure also protects the Types header.

Validation

• 270 Editor unit tests
• 50 Editor Playwright end-to-end tests
• TypeScript project build/typecheck
• production Vite build
• bundle budget and CSP checks
• deployment manifest tests
• new regression demonstrated failing before the fix and passing after it