feat: show click (pointer) cursor on hover over Material controls and progress bar

[Ortes]

Description

On web and desktop, hovering the mouse over the player controls keeps the default arrow cursor — there's no affordance that the buttons and the seek bar are clickable.

The affected controls (play/pause, mute, subtitle toggle, fullscreen, and the draggable progress bar) are built with GestureDetector, which — unlike IconButton — does not set a hover cursor. The options/subtitles IconButtons already show a pointer, so the bar is inconsistent.

This PR wraps the clickable child of each of those GestureDetectors in a MouseRegion(cursor: SystemMouseCursors.click) so the cursor turns into a pointer on hover, matching the IconButton-based controls.

What changed

• MaterialControls — mute, fullscreen, subtitle toggle
• MaterialDesktopControls — play/pause, mute, fullscreen
• VideoProgressBar (shared) — the draggable progress/seek bar only; the non-draggable variant is left untouched

CupertinoControls is intentionally left out: it is touch-first (iOS) and its root already wraps everything in a MouseRegion. Happy to extend it in a follow-up if maintainers prefer full coverage.

Behavior

• No functional or layout change — MouseRegion only affects the hover cursor.
• The pointer region matches the existing tap region exactly (the MouseRegion wraps the same child the GestureDetector already used).

Testing

• flutter analyze — no issues.
• Built and ran the example/ app on web; controls behave identically, with the pointer cursor now appearing on hover over the buttons and the seek bar.

🤖 Generated with Claude Code

[codecov]

Codecov Report

❌ Patch coverage is 86.36364% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 44.00%. Comparing base (69a8fc7) to head (20ff4cc).
⚠️ Report is 2 commits behind head on master.

Files with missing lines	Patch %	Lines
lib/src/material/material_controls.dart	72.72%	6 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #950      +/-   ##
==========================================
+ Coverage   43.82%   44.00%   +0.18%     
==========================================
  Files          21       21              
  Lines        1602     1609       +7     
==========================================
+ Hits          702      708       +6     
- Misses        900      901       +1     

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