build: mark missing overrides and drop unused lambda captures (1,156 clang warnings) (#15334)

* chore: mark every declaration that overrides a base virtual

clang-cl reports 42 member functions across 28 files that override a
base virtual without being marked `override`, inside classes that
already mark their other overrides. That is every occurrence of
-Winconsistent-missing-override in the tree, so the category drops to
zero and -Werror=inconsistent-missing-override becomes available as a
guard against it coming back.

Behaviour is unchanged. Each keyword goes only where clang had already
resolved the declaration to a base virtual, so it records what the
compiler already worked out and cannot affect overload resolution or
dispatch. If any of these signatures had not really overridden a base
method, the build would have failed rather than warned.

Where a declaration already carried `virtual` it is left alone and the
keyword appended, matching the surrounding declarations. Plain
`override` is used rather than the wxWidgets `wxOVERRIDE` macro, which
wx/defs.h defines as `override` beneath a comment marking it obsolete,
and which the rest of src/slic3r already avoids by 1742 occurrences to
113.

A full clang-cl build takes -Winconsistent-missing-override from 1,146
warning lines to 0. Those 42 declarations produce that many lines
because a header is re-diagnosed in every translation unit that
includes it. CalibrationWizardStartPage.hpp alone accounts for 336 of
them from 4 declarations.

* chore: drop unused lambda captures in GUI/Widgets

clang-cl reports 10 lambda captures in src/slic3r/GUI/Widgets that are
never read. Removing them changes nothing at runtime.

Every capture removed is `this` or a raw pointer. clang does not report
a capture whose type has a non-trivial destructor, since such a capture
can be held purely for its effect on an object's lifetime, so nothing
that owns or extends a lifetime is touched. The std::weak_ptr captured
beside the removed `this` in MultiNozzleSync.cpp stays.

This clears the category in GUI/Widgets only. A full clang-cl build
takes -Wunused-lambda-capture from 312 warning lines to 302, leaving
235 sites in other directories for a follow-up.

src/slic3r/GUI/Widgets/WebView.cpp

@@ -104,7 +104,7 @@ DWORD DownloadAndInstallWV2RT() {
 class WebViewEdge : public wxWebViewEdge
 {
 public:
-    bool SetUserAgent(const wxString &userAgent)
+    bool SetUserAgent(const wxString &userAgent) override
     {
         bool dark = userAgent.Contains("dark");
         SetColorScheme(dark ? COREWEBVIEW2_PREFERRED_COLOR_SCHEME_DARK : COREWEBVIEW2_PREFERRED_COLOR_SCHEME_LIGHT);