Widen entry form to fill available window width

[normanajn]

Summary

• The New Entry / Edit Entry form column was hard-capped at max-w-2xl (672px) regardless of window size, leaving a lot of unused whitespace on wider screens.
• Changed both the header block and the form column in apps/entries/templates/entries/form.html to max-w-[160ch] so the form grows to fill available space, capped at 160 columns.
• Applies to both Create and Edit (they share the same template). Small-screen/minimum-width behavior is unchanged.

Closes #23

Test plan

• Verified locally at 500px, 900px, 1400px, and 2200px viewport widths on both New Entry and Edit Entry — form grows from 672px up to ~896px (bound by the existing shared site container + sidebar), stays centered/capped on ultra-wide viewports, and narrow-viewport rendering is unchanged.
• Confirmed the markdown cheat-sheet sidebar still renders correctly alongside the widened form at the lg breakpoint and above.

🤖 Generated with Claude Code

[Copilot]

Pull request overview

This PR widens the “New Entry / Edit Entry” page layout by increasing the maximum width of the header section and the main form column so they better utilize available horizontal space on larger viewports, addressing the request in #23.

Changes:

• Updated the header container from max-w-2xl to max-w-[160ch].
• Updated the main form column container from max-w-2xl to max-w-[160ch] while preserving existing w-full min-w-0 behavior.

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