fix(web): keep shell flex from overflowing at 1280 and 360

[jonathanung]

Summary

• Tokenize shell geometry (--header-height, --gutter, --pane-min, --mode-bar-height) and let desktop columns shrink instead of forcing a 420px canvas min.
• Header/wordmark/palette, rails, inspector, transcript, composer, Code explorer, Team, and settings wrap or shrink so the page does not scroll horizontally.
• Phone mode bar stays visible above sheets (the previous hide rule won the cascade). Header export hides at 360; it remains on the composer.

Issue

Fixes #1236

Tier

B

Verification

• make web-check (334 tests)
• make web-e2e (18 passed), including desktop-1280 and narrow-360 Chat + inspector + Code/Team
• smoke: skipped (web CSS/layout only)

Layout check

• Playwright headless Chromium at 1280×800 and 360×640: no page-level h-scroll; settings, send, inspector, Code/Team modes reachable.
• Could not headed-click (no interactive display); verification is headless Playwright plus CSS contract tests.

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[jonathanung]

Review pass 1 (head eec6217e1da2d35c773ca083afe14103b1a2b41c) — clean

0 blocking, 0 should-fix.

In-process strike-reviewer rubric on gh pr diff 1243 plus surrounding CSS callers/tests.

• Desktop canvas no longer has a 420px min, so nav+inspector resize cannot force page overflow.
• Phone mode-bar cascade is fixed and asserted (hide rule cannot win).
• Playwright covers 1280 and 360 Chat + inspector + Code/Team with tightened page-scroll slack.

Nits (deferred): --pane-min is documented but unused by a rule; header export hide is aria-label-coupled. Neither is a failure scenario on this SHA.

Merge checklist may proceed for this SHA only.