fix(frontend): repair mobile admin layout regressions

[kirafishy]

Summary

• keep the mobile navigation sheet account controls visible at the bottom on short viewports
• constrain the settings tab list to the page width and allow horizontal scrolling on narrow screens
• stop the settings sticky header overlay from covering the mobile top navigation chrome

Root cause

1. Missing bottom sidebar actions
   The mobile SheetContent used flex column layout without a viewport height cap. Long navigation content expanded past the screen and pushed the account/settings footer out of view.

2. Settings tabs overflow
   Settings tabs were horizontal with every trigger set to w-full, so the tab list grew wider than the viewport and was not clipped or scrollable.

3. Invisible but clickable top controls
   The settings sticky header used a full-viewport before background mask and top-8, which covered the mobile shell header visually while leaving hit targets active. The shell header itself was also not sticky.

Changes

• app-shell.tsx: pin the mobile sheet to h-dvh, scroll only the nav region, keep account controls shrink-0, and make the mobile shell header sticky
• settings-page.tsx: limit tab list width with horizontal overflow scrolling; keep desktop vertical sticky tabs; move the scroll mask to lg: only and align sticky offset with the mobile header

Impact

• Mobile administrators can reach account, theme, language, and settings controls from the drawer again
• Settings tabs stay within the page width on phones
• Mobile menu / title / GitHub controls remain visible while scrolling the settings page
• Desktop layout behavior is intentionally preserved

Validation

• pnpm run build
• pnpm exec tsc -p tsconfig.app.json --noEmit
• Local backend + Vite smoke test on mobile viewport:
  • dashboard drawer shows bottom account controls
  • settings tabs no longer overflow the page
  • settings page top chrome remains visible while scrolling