fix(collab-web): show document names and avoid early header wrapping

[OTime-Coder]

Review Headers show the Worktree name instead of the selected document, and reserve 260px for the title even when a short filename needs much less space. This can drop actions onto a second row despite enough room for all controls.

Use the selected Unit name and measure the title row's actual minimum width, keeping the icon, badge, sidebar control, and internal gaps intact. Only long filenames may shrink to their CSS minimum. Use the same measured width for geometric centering and the Flexbox basis; the browser still handles all wrapping. Measure an uncompressed row before paint so narrow containers cannot corrupt the result.

Keep lifecycle commands bound to the active Worktree, preserve complete merge-status messages, and update the fixture, regression tests, and bilingual README.

Validation: full pnpm check passed, including 143 collab-web tests; explicit Header test/fixture typecheck passed. 248 browser scenarios cover Chinese/English, short and long names, normal/preview and lifecycle states, sidebar-toggle spacing, dark mode, and exact centering/wrapping boundaries. Checks assert that controls never wrap when their actual minimum widths fit, in addition to bounds, order, and centering checks. git diff --check passed.