Give every admin page consistent padding and contain page crashes

[man4ish]

Why

Admin pages rendered straight into the shell with no padding:

• Flush layout: every title, description and card sat against the sidebar, directly under the top bar, and ran to the right edge of the window. Measured on a local build, every page title was 0px from the sidebar and 0px from the top bar.
• Misaligned placeholders: only the "Coming soon" placeholder used PageContainer, which centered it in a 1280px box, so those pages were aligned differently from everything else.
• Crashes blank the console: a single page render error blanked the whole console, sidebar and top bar included, until a reload.

What

• AppShell: wraps every page in <main className="shell-content"> with 24px/32px padding, full width and left-aligned. The existing 560px rule still drops the padding to 16px on phones.
• PageContainer: no longer adds padding or centering, so nothing is padded twice and the placeholders line up with every other page.
• PageErrorBoundary (new, keyed by the active page): a render error now shows an in-page error with a Retry button, and the sidebar and top bar keep working.
• Search box: the disabled "Search (coming soon)" box is hidden below 1700px, where it squeezed the breadcrumb to "Administrat… › Us…".

Checks

• Every sidebar page, local admin build:
  • 1600px: titles sit 32px from the sidebar and 24px below the top bar.
  • 1280px: no horizontal overflow.
• New tests: the shell pads pages in one place and doesn't center them; a crashing page keeps the shell and can retry.
• Results: vitest 748 passed; tsc -b, build:admin and build:control are clean.

🤖 Generated with Claude Code

https://claude.ai/code/session_014t7uDmnjGZAyMrCYpN2RZy

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Generated by Claude Code