fix: make diagram sidebar more compact for iOS app

[tamang29]

Related issue: #779

Summary

• Compact the diagram element sidebar on mobile viewports.
• Reduce whitespace between palette entries in portrait and landscape.
• Preserve the 44px minimum touch target and existing desktop layout.
• Automatically update the layout when device orientation changes.
• Add unit and E2E regression coverage.

Release note

See more of the canvas on mobile with a compact element palette in both portrait and landscape.

Implementation notes

Screenshots / screencasts

Checklist

• Linked to a related issue (if applicable)
• Added a changeset whose summary is written in the user's voice (pnpm changeset, how) — or this PR doesn't touch a Changesets-tracked package (@tumaet/apollon, @tumaet/webapp, @tumaet/server)
• PR title's Conventional Commit type (feat/fix/…) matches the kind of change — it groups the release note
• Tests added or updated
• Documentation updated (if applicable)
• Screenshots or screencasts attached (if a UI change)