feat(settings): add reusable mobile-first settings UI kit

[KucharczykL]

Summary

• add the reusable settings scaffold, grouped Django form fields, supported one/two/three-column layouts, live-save behavior, masked secrets, source/lock metadata, and shared tooltip definition lists
• add the authenticated DEBUG-only settings UI kit preview and codify its visual/component contracts for future settings pages
• replace the ambiguous mobile priority-plus menu with a self-explanatory native-dialog bottom sheet while preserving the same section-link DOM in the desktop sticky rail
• add reusable bottom-sheet controller/surface behavior with focus containment, safe backdrop dismissal, exact scroll restoration, responsive cleanup, and no-JavaScript navigation fallback
• strengthen neutral badge visibility and enforce a cross-tone contract so tone definitions can change palette only, not border, spacing, typography, or shape

Verification

• DEBUG=true make check
• Ruff and formatting: passed
• mypy: 201 source files passed
• Vitest: 573 passed
• pytest and Playwright: 1,919 passed
• generated element types and icons: current

Notes

• the complete section-link list is moved, never cloned, between the mobile sheet and desktop rail
• settings navigation retains ordinary nav/list/link semantics and does not inherit ARIA-menu roles or roving tabindex
• unlocked source badges remain subject to the documented final epic deletion gate if no concrete shipped use case survives

Closes #384

[github-actions]

Staging deployment: https://timetracker-staging-codex-384-settings-ui-kit.fly.dev