feat: show a hover-time indicator on the progress bar

[Ortes]

Description

Adds a small floating time indicator to the Material progress bar. While the pointer hovers (or drags) the bar, a pill shows the timecode at that position, so the user can see exactly where a click or seek will jump to — the behaviour people expect from YouTube/Netflix-style scrubbers.

The pill is rendered as an overlay above the bar and follows the pointer horizontally, clamped to the bar's width, with a small caret pointing at the exact spot. It uses the existing formatDuration helper, so it matches the position / duration label already shown in the controls.

Behaviour

• Only shows once the video is initialized and its duration is known.
• Driven by hover events, so it never appears on touch devices (which don't emit hover) — mobile behaviour is unchanged. It does also track an in-progress drag, which gives touch users seek feedback while scrubbing.
• Purely additive and self-contained in progress_bar.dart; no public API changes.

Notes

Scoped intentionally to the hover-time readout only — it does not set the hover cursor (that is handled separately in #950).

[codecov]

Codecov Report

❌ Patch coverage is 11.11111% with 72 lines in your changes missing coverage. Please review.
✅ Project coverage is 42.71%. Comparing base (69a8fc7) to head (c0f2c74).
⚠️ Report is 2 commits behind head on master.

Files with missing lines	Patch %	Lines
lib/src/progress_bar.dart	11.11%	72 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #956      +/-   ##
==========================================
- Coverage   43.82%   42.71%   -1.11%     
==========================================
  Files          21       21              
  Lines        1602     1660      +58     
==========================================
+ Hits          702      709       +7     
- Misses        900      951      +51     

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