fix: stabilize pagination nav width to prevent layout shift on page change

[octo-patch]

Fixes #1363

Problem

The pagination component's <nav> element changes width when navigating between pages, causing a visible layout "jump". This happens because the number of rendered page buttons is variable:

• Page 1 of 23: « 1 2 3 … 23 » → 5 page items
• Page 6 of 23: « 1 … 4 5 6 7 8 … 23 » → 9 page items

The old visiblePages algorithm used page ± 2 as a sliding window, producing between 5 and 9 items depending on the current page, causing the nav to grow and shift other layout elements.

Solution

Replace the variable-width algorithm with one that always returns exactly 7 items when totalPages > 7:

Condition	Result (7 items)
page ≤ 4	1 2 3 4 5 … total
page ≥ total - 3	1 … t-4 t-3 t-2 t-1 t
middle	1 … p-1 p p+1 … total

This ensures the <nav> maintains a constant width regardless of which page the user is on, eliminating the layout shift.

Testing

Verified the output length of visiblePages is stable across all pages for a 23-page dataset by manually tracing the algorithm:

• Page 1: [1,2,3,4,5,'...',23] → 7 items ✓
• Page 5: [1,'...',4,5,6,'...',23] → 7 items ✓
• Page 12: [1,'...',11,12,13,'...',23] → 7 items ✓
• Page 20: [1,'...',19,20,21,22,23] → 7 items ✓
• Page 23: [1,'...',19,20,21,22,23] → 7 items ✓