fix: add mounted check in _startHideTimer to prevent setState after dispose

[Cleparr]

Problem:
_startHideTimer calls setState inside a Timer callback without checking mounted first. If the widget is disposed before the timer fires (e.g. user navigates away), this causes a fatal crash: Null check operator used on a null value.

Fix:
Add if (!mounted) return; before setState, following the same pattern already used in _bufferingTimerTimeout and _updateState (PR #470).

[codecov]

Codecov Report

❌ Patch coverage is 0% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 43.82%. Comparing base (d228479) to head (bec9326).
⚠️ Report is 4 commits behind head on master.

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

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #940      +/-   ##
==========================================
- Coverage   43.84%   43.82%   -0.03%     
==========================================
  Files          21       21              
  Lines        1601     1602       +1     
==========================================
  Hits          702      702              
- Misses        899      900       +1     

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[diegotori]

LGTM.