refactor(sidebar): use external-store viewport subscriptions

[IzumiSy]

Motivation

The sidebar's mobile and tablet hooks currently subscribe to matchMedia and copy the viewport into React state through effects. Both initially return false, so client-only rendering starts with desktop assumptions even when the actual viewport is mobile or tablet.

The effects synchronously update the viewport state after the first commit, triggering an additional render solely to copy a browser-owned value into React state.

This extracts the responsive-hook changes from #525 into an independent PR based on main, without the lint upgrade or the other state-ownership changes.

Design Decision

Subscribe directly to the browser-owned viewport

Use useSyncExternalStore in the two existing hooks, retaining their current innerWidth predicates and media-query subscriptions. Module-level subscription functions remain stable across rerenders and remove the same listeners on unmount. No shared hook abstraction or public API is added.

Preserve server rendering and hydration

Both server snapshots remain false. Server rendering and the first hydration render therefore retain the existing desktop fallback; client-only rendering reads the actual viewport immediately.

The desktop Sidebar test fixture moves from 1024px, which lies in the tablet range, to 1200px. A separate 1024px assertion retains coverage of the tablet icon rail rather than relying on the temporary desktop fallback.

Summary

• Replace effect-mirrored state in the mobile/tablet hooks with stable external-store subscriptions.
• Cover breakpoint boundaries, first client render, change notifications, subscription cleanup, SSR, and hydration.
• Correct the desktop Sidebar fixture, explicitly cover tablet layout, and add a patch changeset for the initial-render behavior change.

[github-actions]

Code Metrics Report

	main (d8d4ec3)	#602 (4194321)	+/-
Coverage	88.6%	88.6%	+0.0%
Test Execution Time	1m53s	1m31s	-22s

Details

  |                     | main (d8d4ec3) | #602 (4194321) |  +/-  |
  |---------------------|----------------|----------------|-------|
+ | Coverage            |          88.6% |          88.6% | +0.0% |
  |   Files             |            208 |            208 |     0 |
  |   Lines             |           6263 |           6260 |    -3 |
- |   Covered           |           5552 |           5551 |    -1 |
+ | Test Execution Time |          1m53s |          1m31s |  -22s |

Code coverage of files in pull request scope (90.4% → 100.0%, patch 100.0%)

Files	Coverage	+/-	Patch Coverage	Status
packages/core/src/hooks/use-mobile.ts	100.0%	+10.0%	100.0%	modified
packages/core/src/hooks/use-tablet.ts	100.0%	+9.0%	100.0%	modified

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Reported by octocov

[IzumiSy]

/review

[github-actions]

✅ Code Review completed successfully!

Code review complete for PR #602 package changes. No actionable High/Medium issues found in the packages/** diff, so no inline review comment was needed.

Generated by Code Review for #602