chore: raise PHPStan to level 10

[dakorpar]

Follow-up to #134, and the last step — 10 is PHPStan's maximum level. It treats implicit mixed the same way level 9 treats explicit mixed.

All 17 errors had the same origin: reading back through Nette's magic accessors, which are mixed by nature.

The class[] idiom, in one place

Nine of the errors were $el->class[] = '...' — spread across seven files, each preceded (or, in two cases, not preceded) by its own BootstrapUtils::standardizeClass() call. It worked because Nette's Html::__get() returns by reference, so the append auto-vivified the attribute.

BootstrapUtils now carries that knowledge instead:

BootstrapUtils::fetchClasses($el);       // classes as a list, however the attribute was set
BootstrapUtils::addClass($el, 'form-control');
BootstrapUtils::removeClass($el, 'ajax');
BootstrapUtils::hasClass($el, 'ajax');

Call sites now read as what they do. BootstrapRenderer::fetchClasses(), which #134 added, was this same logic a second time and now delegates to the util. standardizeClass() is kept — it is public API — though nothing in src/ needs it any more.

The rest

• BootstrapForm::setAjax() did the class juggling by hand (standardizeClass → read → in_array → array_diff → write back). It goes through the util now and keeps its "only add if absent" behaviour, which a new test pins down.
• The onError handler took an untyped $form with @param BootstrapForm $form in a docblock positioned where PHPStan could not attach it. It is a real parameter type now.
• renderBegin() read $el->action three times, casting mixed to string and carrying two @noinspection comments. It reads the attribute once into a local and normalizes it there — which also handles an action set as a Nette\Application\UI\Link rather than a plain string.
• addInputError() called addError() on whatever getComponent() returned. It now checks for a BaseControl and throws InvalidArgumentException naming the component when given something that cannot hold an error (a container, say), instead of dying on an undefined method.
• UploadInput loses trim($control->class .= ...), which relied on the class attribute being a string and on .= evaluated inside a function call.

Verification

• make phpstan (now -l 10, the maximum) — no errors
• make cs — clean
• make tests — 181 tests, 277 assertions, all passing

8 new tests: six cover the new BootstrapUtils methods (both attribute shapes, absent attribute, removing a class that is not there), one that setAjax(true) twice does not duplicate the class, one for the addInputError guard.

Rendered HTML is unchanged — no fixture updates. The snapshot suite is the safety net for the class[] refactor: every one of those seven files feeds into fixtures that still match byte for byte.

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.54%. Comparing base (faea76b) to head (4790aa6).

Additional details and impacted files

@@             Coverage Diff              @@
##             master     #135      +/-   ##
============================================
+ Coverage     97.51%   97.54%   +0.02%     
- Complexity      333      340       +7     
============================================
  Files            25       25              
  Lines          1007     1019      +12     
============================================
+ Hits            982      994      +12     
  Misses           25       25              

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