chore: raise PHPStan to level 8

[dakorpar]

Raises the PHPStan level in make phpstan from 7 to 8. Level 8 checks method calls and property access on nullable types, which surfaced three real holes rather than just noise.

What level 8 found

BootstrapRenderer::configElem() could throw away the element it was configuring.
When a config entry had a CONTAINER key whose own config produced no element, $el = $container overwrote the element with null and the caller silently lost it. It now only swaps in the container when there is one. That also makes the method total — given an element, it always returns one — which is expressed with a conditional return type:

@return ($el is null ? Html|null : Html)

With that, getElem() is declared : Html instead of : ?Html, and its ~20 call sites across renderBody(), renderPair(), renderFeedback() and renderDescription() stop looking nullable. Two configElem(…, null) calls that always needed a real element were switched to getElem(), and a dead !empty($nonLabel) guard on an always-truthy Html object was dropped.

BootstrapRow::$container was typed Container but could hold null.
setParent() takes ?IContainer and assigned straight to the property, so a detached row (or one attached to a non-Nette\Forms\Container) violated its own declared type. Now:

• the property is Container|null;
• getParent() returns ?IContainer, matching IComponent and reality;
• setParent() throws InvalidArgumentException for a parent that is not a Nette\Forms\Container, instead of storing something the rest of the class cannot use;
• a new getContainer(): Container gives render(), addComponent() and BootstrapCell::render() the attached container, or an InvalidStateException with a clear message instead of a null-call fatal.

BootstrapRow::$ownedNames was declared string[] but collected ?string.
It now records the name the component actually ends up with after the add.

API notes

BootstrapRow::getParent() widened from IContainer to ?IContainer. Callers that need the container should use the new getContainer().

Verification

• make phpstan (now -l 8) — no errors
• make cs — clean
• make tests — 161 tests, 245 assertions, all passing (3 new tests cover the detached-row and wrong-parent paths)

No fixture changes: rendered HTML is unchanged.

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.48%. Comparing base (87fa3b3) to head (1a19a7f).
⚠️ Report is 2 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff              @@
##             master     #133      +/-   ##
============================================
+ Coverage     97.05%   97.48%   +0.43%     
- Complexity      309      313       +4     
============================================
  Files            25       25              
  Lines           986      996      +10     
============================================
+ Hits            957      971      +14     
+ Misses           29       25       -4     

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