fix: make dashboard design tokens contrast-safe

[icebear0828]

Summary\n\n- add a self-contained DESIGN.md for the Codex Proxy dashboard visual system\n- split readable primary text/container tokens from solid primary action tokens and map status/avatar foreground-container pairs through Tailwind\n- migrate repeated dashboard buttons, status chips, avatars, toggles, and active tab/filter states to semantic tokens\n- fix mobile dashboard tab wrapping and make AccountList storage access use window.localStorage for browser/jsdom consistency\n\nCloses #497.\n\n## Validation\n\n- npm run build\n- cd web && npx vitest run\n- npx vitest run tests/unit/web/theme.test.ts\n- npx @google/design.md lint DESIGN.md -> errors: 0, warnings: unused token references only\n- Headless Chrome visual check via Vite at 127.0.0.1:5175: overview light/dark desktop, usage dark desktop, settings light desktop, accounts dark mobile; mobile accounts horizontal overflow fixed from 211px to 0px\n\n## Known base issue\n\n- Full npm test is still blocked by #505: tests/unit/update-scripts-path.test.ts expects missing scripts/build/check-update.ts and scripts/build/apply-update.ts. Reproduced independently after rebasing onto latest origin/dev.\n

[icebear0828]

Extended browser smoke completed after the latest push (9099e85).

What changed after the first broad pass:

• The full matrix initially found page-level horizontal overflow on #/logs at 390px mobile in both light and dark modes.
• Fixed LogsPage by stacking the list/details panes on mobile and constraining the log table to an internal horizontal scroll container.
• Added a LogsPage regression test for the responsive table/detail layout.

Browser smoke matrix (http://127.0.0.1:5175/, backed by local 8080 proxy):

• Pages: #/, #/accounts, #/api-keys, #/proxies, #/usage-stats, #/logs, #/errors, #/settings
• Viewports/themes: 1440x1000 light, 1440x1000 dark, 390x900 light, 390x900 dark
• Result: 32 / 32 passed
• Checks per case: #app rendered, main rendered, non-empty text, active tab computed foreground/background present, theme class correct, runtime exceptions = 0, console errors = 0, page-level horizontal overflow = 0
• Screenshot + JSON evidence: /tmp/codex-proxy-ui-shots/full-smoke-rerun-2026-05-12T11-23-41-443Z/summary.json

Validation:

• cd web && npx vitest run -> pass, 4 files / 11 tests
• npx vitest run tests/unit/web/theme.test.ts -> pass, 14 tests
• npm run build -> pass
• npx @google/design.md lint DESIGN.md -> 0 errors, 62 existing unused-token warnings

Known blocker outside this PR:

• Full root npm test is still blocked by test: update script path check references missing build scripts #505 (tests/unit/update-scripts-path.test.ts expects missing update scripts). I did not fold that unrelated base issue into this dashboard PR.