fix(webviews): keep model picker rail tabs round
The model picker harness rail rendered as rounded squares after #25796 changed its button radius to 6px. Restoring a 50% radius keeps every tab circular, including the selected tab, while preserving the existing equal-size rail layout.
Changelog
- Fixed: keep model picker harness rail tabs round.
Verification
- Red at
e6525bd5f17:CMUX_BROWSER_TESTS=1 bun test test/model-picker-round-rail.test.ts— Chromium failed the existing round-rail assertion for all 14 tabs. - Green at
c2c0a356ea9: same command — Chromium passed (1 pass,0 fail; WebKit unavailable on the test host). python3 scripts/cmux-next/prepush-gate.py— no new violations.
Observed / Not run
Observed: the focused browser regression passes after the one-line CSS fix, and the independent review shipped the exact head.
Not run: WebKit browser coverage was unavailable locally. The web lint wrapper is also blocked by the base tree's stale generated src/agent-session/acpmux/protocol/generated/methods.ts, unrelated to this CSS-only change.
Agent-Session: c2c0a356ea984bfb8d9b1114244d4740c1a637f1
Agent session: agent session
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Summary by cubic
Restores fully round tabs in the model picker harness rail, which had rendered as rounded squares since #25796 changed the button radius to 6px. Switching border-radius back to 50% keeps every tab circular, including the selected tab, while preserving the existing equal-size rail layout.