feat: YouTube-style seek indicator for keyboard seeking

[Ortes]

What

Adds a YouTube-style seek indicator that flashes on screen when the user seeks with the keyboard arrows on the desktop controls (MaterialDesktopControls).

• Pressing → / ← shows a small pill with a directional icon and the amount seeked.
• Repeated presses in the same direction keep the pill visible and accumulate the total (e.g. 10s → 20s → 30s), just like YouTube.
• Pressing the opposite direction (or after the pill fades ~0.9s) resets the counter.

How

• New reusable widget lib/src/seek_indicator.dart (fades via AnimatedOpacity, mirrors the existing CenterSeekButton style; purely visual — wrapped in IgnorePointer).
• _MaterialDesktopControlsState accumulates the seeked seconds in _seekForward/_seekBackward (kept in sync with the existing Duration seek step) and auto-hides via a Timer, mirroring the existing buffering-indicator pattern.
• Gated by a new ChewieController.showSeekIndicator option (default true), wired through copyWith.

Tests

test/seek_indicator_test.dart covers accumulation on repeated presses, direction reset, auto fade-out, the showSeekIndicator: false opt-out, and the option default/copyWith.

dart format, flutter analyze lib test, and the full flutter test suite are clean.

[codecov]

Codecov Report

❌ Patch coverage is 97.61905% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 50.97%. Comparing base (69a8fc7) to head (637e2a7).
⚠️ Report is 2 commits behind head on master.

Files with missing lines	Patch %	Lines
lib/src/chewie_player.dart	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #953      +/-   ##
==========================================
+ Coverage   43.82%   50.97%   +7.15%     
==========================================
  Files          21       22       +1     
  Lines        1602     1642      +40     
==========================================
+ Hits          702      837     +135     
+ Misses        900      805      -95     

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[Ortes]

Hi @diegotori 👋

Beyond this PR, I'd like to offer to help with maintenance on chewie if you're open to it.

A bit of context:

• I've had PRs merged here before (Add keyboard controls seek forward and backward and fullscreen escape on desktop #876 desktop keyboard controls, Fix example on web #910 web example fix), and just opened three more, each with tests, a CHANGELOG entry and green CI: feat: YouTube-style seek indicator for keyboard seeking #953 (keyboard seek indicator), feat: auto-hide the mouse cursor while idle in fullscreen #954 (auto-hide cursor in fullscreen), feat: toggle fullscreen with the F key on desktop #955 (F-key fullscreen toggle).
• I use chewie in production at my company, and also personally on a daily/weekly basis inside a custom movie player I built on top of it — so I'm exercising it heavily across web and mobile.

I know the open-PR queue is long, and I'd be glad to take some of that load off you: triaging and reviewing PRs, keeping the package current with new Flutter releases, and helping land fixes. Would you be open to adding me as a co-maintainer (or triage access to start)? Happy to do whatever works best for you and the Flutter Community org.

Either way, thanks for maintaining this 🙏