ui: migrate dashboard shell and account workflows to shared primitives

[icebear0828]

Context

The dashboard shell and account views carry the highest UI reuse pressure: sticky header, top tabs, account cards, account management, import/export toolbar, proxy assignment controls, status badges, toggles, quota bars, and destructive icon actions. These are also the places where contrast, hover, wrapping, and dark-mode consistency matter most.

Depends on #497 and #498.

Scope

• Migrate Header actions, language/theme controls, update controls, server status pill, and add-account CTA to shared primitives.
• Migrate the top tab bar to the shared navigation primitive.
• Migrate AccountCard status badge, toggle, icon actions, proxy select, quota bars, and dense metric rows.
• Migrate AccountList and AccountManagement toolbars/filters/import-export actions where they share the same control patterns.
• Preserve current dense layout and account semantics. Do not swap history/window/summary values for visually convenient alternatives.

Acceptance criteria

• Header, tabs, account list/cards, and account management screens visually match DESIGN.md in light and dark modes.
• Existing account actions, selection, import/export, refresh, status toggle, label edit, proxy assignment, and quota refresh behavior still work.
• Narrow-screen wrapping remains safe; no toolbar/card overflow.
• No TypeScript any is introduced.

Verification

• Add/update tests for migrated account UI behavior and class/state regressions.
• Run npm test.
• Run npm run build.
• Manually verify the overview and account management tabs at localhost:8080 in light and dark mode.

Notes

This issue should stay focused on shell/account migration. Usage charts, logs, errors, proxy-pool detail screens, and settings panels can be handled separately.