feat(core): add seek button component

[mihar-22]

Closes #495

Summary

Add SeekButton component for seeking forward/backward by a configurable number of seconds. Follows the same three-layer pattern as PlayButton (Core + HTML + React) with a single seconds prop that supports positive (forward) and negative (backward) values.

Changes

• SeekButtonCore class with seconds prop (default 30), label generation using Math.abs to avoid negative signs in accessibility labels
• SeekButtonDataAttrs mapping seeking and seconds to data attributes for CSS-driven styling
• <media-seek-button> custom element with seconds, label, and disabled attributes, using selectTime selector
• <SeekButton> React component with render prop support, matching existing button patterns
• 19 unit tests covering props, state projection, labels, attrs, and seek action

Implementation details

Design decision: Single component vs two. Unlike Media Chrome (separate forward/backward elements), this uses Vidstack's approach — one component where the sign of seconds communicates direction. Avoids ~95% code duplication while data-seconds enables CSS targeting via [data-seconds^="-"].

No clamping in the button. The store's timeFeature.seek() already clamps to [0, duration], so the button just computes currentTime + seconds and delegates.

Default 30s matches both Vidstack and Media Chrome industry defaults.

Testing

pnpm -F @videojs/core test src/core/ui/seek-button

All workspace tests pass: pnpm test (12 tasks, 244+ tests).

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[github-actions]

📦 Bundle Size Report

Package	Size	Diff		%
@videojs/core	4.20 kB	+122 B	████████	+2.9%	🔺
@videojs/element	1.60 kB	0 B	░░░░░░░░	0%	✅
@videojs/html	7.67 kB	+163 B	██████░░	+2.1%	🔺
@videojs/react	6.81 kB	+197 B	████████	+2.9%	🔺
@videojs/store	1.86 kB	0 B	░░░░░░░░	0%	✅
@videojs/utils	1.87 kB	0 B	░░░░░░░░	0%	✅

Total: 24.01 kB · +482 B · +2.0%

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Entry Breakdown

Subpath sizes are the additional bytes on top of the root entry point, measured by bundling root + subpath together and subtracting the root-only size.

@videojs/core

Entry	Base	PR	Diff	%
.	1.39 kB	1.52 kB	+131 B	+9.2%	🔺
./dom	2.68 kB	2.67 kB	-9 B	-0.3%	🔽
total	4.08 kB	4.20 kB	+122 B	+2.9%

@videojs/element

Entry	Base	PR	Diff	%
.	817 B	817 B	0 B	0%	✅
./context	823 B	823 B	0 B	0%	✅
total	1.60 kB	1.60 kB	0 B	0%

@videojs/store

Entry	Base	PR	Diff	%
.	1.29 kB	1.29 kB	0 B	0%	✅
./html	394 B	394 B	0 B	0%	✅
./react	190 B	190 B	0 B	0%	✅
total	1.86 kB	1.86 kB	0 B	0%

@videojs/utils

Entry	Base	PR	Diff	%
./array	104 B	104 B	0 B	0%	✅
./dom	553 B	553 B	0 B	0%	✅
./events	227 B	227 B	0 B	0%	✅
./function	181 B	181 B	0 B	0%	✅
./object	119 B	119 B	0 B	0%	✅
./predicate	265 B	265 B	0 B	0%	✅
./time	465 B	465 B	0 B	0%	✅
total	1.87 kB	1.87 kB	0 B	0%

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ℹ️ How to interpret

Sizes are minified + brotli, measured with esbuild.
Package totals are computed as root size + marginal subpath costs.
Subpath marginal cost = (root + subpath bundled together) − root alone.

Icon	Meaning
✅	No change
🔺	Increased ≤ 10%
🔴	Increased > 10%
🔽	Decreased
🆕	New (no baseline)

Run pnpm size locally to check current sizes.