fix(UI): Fix ad marker alignment regression

[avelad]

Fixes a UI alignment bug where percentage-based ad markers and interstitials were shifted horizontally. This regression was introduced in PR #10097 due to how CSS handles 'background-position' when using percentage values.

Now, the seek bar uses color stops within a full-width linear gradient for dynamic percentage markers to guarantee exact boundaries, while preserving the original centering logic for absolute-width chapter markers.

[shaka-bot]

Bundle Size Report for PR #10193

File	HEAD	Base	Diff
controls.css	37.2 KiB (6.7 KiB)	37.2 KiB (6.7 KiB)	0.0 KiB
controls.modern.css	29.6 KiB (6.1 KiB)	29.6 KiB (6.1 KiB)	0.0 KiB
demo.compiled.debug.js	263.6 KiB (53.6 KiB)	263.6 KiB (53.6 KiB)	0.0 KiB
demo.compiled.js	263.6 KiB (53.6 KiB)	263.6 KiB (53.6 KiB)	0.0 KiB
demo.css	160.2 KiB (25.7 KiB)	160.2 KiB (25.7 KiB)	0.0 KiB
demo.modern.css	144.1 KiB (24.7 KiB)	144.1 KiB (24.7 KiB)	0.0 KiB
locales.js	44.1 KiB (10.9 KiB)	44.1 KiB (10.9 KiB)	0.0 KiB
receiver.compiled.debug.js	149.2 KiB (24.2 KiB)	149.2 KiB (24.2 KiB)	0.0 KiB
receiver.compiled.js	149.2 KiB (24.2 KiB)	149.2 KiB (24.2 KiB)	0.0 KiB
shaka-player.compiled-es2021.debug.js	1276.3 KiB (314.6 KiB)	1276.3 KiB (314.6 KiB)	0.0 KiB
shaka-player.compiled-es2021.js	675.0 KiB (225.7 KiB)	675.0 KiB (225.7 KiB)	0.0 KiB
shaka-player.compiled.debug.js	1552.4 KiB (356.2 KiB)	1552.4 KiB (356.2 KiB)	+0.0 KiB
shaka-player.compiled.js	780.4 KiB (254.0 KiB)	780.4 KiB (254.0 KiB)	0.0 KiB
shaka-player.dash-es2021.debug.js	943.0 KiB (237.5 KiB)	943.0 KiB (237.5 KiB)	0.0 KiB
shaka-player.dash-es2021.js	469.2 KiB (160.4 KiB)	469.2 KiB (160.4 KiB)	0.0 KiB
shaka-player.dash.debug.js	1145.1 KiB (268.1 KiB)	1145.1 KiB (268.1 KiB)	+0.0 KiB
shaka-player.dash.js	544.9 KiB (180.8 KiB)	544.9 KiB (180.8 KiB)	0.0 KiB
shaka-player.experimental-es2021.debug.js	1670.5 KiB (401.1 KiB)	1670.3 KiB (401.0 KiB)	+0.2 KiB
shaka-player.experimental-es2021.js	932.9 KiB (296.7 KiB)	932.7 KiB (296.6 KiB)	+0.2 KiB
shaka-player.experimental.debug.js	2012.8 KiB (452.1 KiB)	2012.6 KiB (452.1 KiB)	+0.2 KiB
shaka-player.experimental.js	1061.8 KiB (330.8 KiB)	1061.6 KiB (330.8 KiB)	+0.2 KiB
shaka-player.hls-es2021.debug.js	973.8 KiB (243.1 KiB)	973.8 KiB (243.1 KiB)	0.0 KiB
shaka-player.hls-es2021.js	502.7 KiB (170.8 KiB)	502.7 KiB (170.8 KiB)	0.0 KiB
shaka-player.hls.debug.js	1181.1 KiB (274.4 KiB)	1181.1 KiB (274.4 KiB)	+0.0 KiB
shaka-player.hls.js	581.2 KiB (192.1 KiB)	581.2 KiB (192.1 KiB)	0.0 KiB
shaka-player.transmuxer-worker.debug.js	198.9 KiB (48.3 KiB)	198.9 KiB (48.3 KiB)	-0.0 KiB
shaka-player.transmuxer-worker.js	83.8 KiB (27.7 KiB)	83.8 KiB (27.7 KiB)	0.0 KiB
shaka-player.ui-es2021.debug.js	1587.9 KiB (383.6 KiB)	1587.7 KiB (383.5 KiB)	+0.2 KiB
shaka-player.ui-es2021.js	892.3 KiB (285.1 KiB)	892.1 KiB (285.1 KiB)	+0.2 KiB
shaka-player.ui.debug.js	1900.1 KiB (430.7 KiB)	1899.9 KiB (430.6 KiB)	+0.2 KiB
shaka-player.ui.js	1009.7 KiB (316.6 KiB)	1009.5 KiB (316.6 KiB)	+0.2 KiB

[shaka-bot]

Incremental code coverage: 11.11%