Add crash-time ring flush and a diagnostics export UI (discussion #644, steps 4-5)

Stacked on the steps 1-3 branch (feature-diagnostic-logging, PR #646).
Kept as its own PR rather than folded into that one, matching the
discussion's own framing: step 4 is explicitly "the highest-risk piece
... lands last, behind its own switch."

## Step 4 -- crash-time ring flush (CrashHandler)

Installs a handler for SIGSEGV/SIGABRT/SIGBUS/SIGFPE/SIGILL (POSIX) /
SetUnhandledExceptionFilter (Windows) that flushes the in-memory ring to
a fixed crash_dump.log before the process dies.

This required reworking LogRing (step 3) to be genuinely lock-free, not
just mutex-protected: a signal handler that blocks on a lock the
crashing thread (or another thread) already holds turns a clean crash
into a hang -- no ring dump *and* no core dump, worse than doing
nothing. append() now claims a slot with a single atomic fetch-add;
dumpToFd() reads the preallocated entries directly and writes them with
write(2) only, looping on EINTR/short writes. Accepted tradeoff: at most
one entry can be read torn if a crash lands mid-append into that exact
slot -- documented in logring.h, and the alternative (a seqlock to
detect and retry) wasn't judged worth the complexity for that window.

Other invariants implemented per the discussion:
- sigaltstack with a static 64 KiB buffer, SA_ONSTACK -- a stack-
  overflow SIGSEGV has no usable stack for a handler without one.
- Nothing under the actual handler touches Qt, QString or the
  allocator: the dump path and a small header (version/git/OS/Qt) are
  precomputed into fixed char buffers by install(), which runs once at
  startup in normal context.
- Atomic test-and-set so only the first crash writes a dump; a second
  concurrent/nested fault goes straight to restore-and-re-raise.
- After writing, the handler restores SIG_DFL and re-raises (POSIX) /
  returns EXCEPTION_CONTINUE_SEARCH (Windows) so the OS's own crash
  path -- core dump, Windows Error Reporting -- still runs. A handler
  that "fixed" the crash by swallowing the signal would destroy exactly
  the post-mortem evidence this whole design exists to preserve.

Tested in this environment: POSIX/Linux only, all five signals. Sent
each directly to a running process and confirmed (a) crash_dump.log is
written with the correct header and ring contents, mode 0600, and (b)
the process still terminates via the signal with the kernel's own
"core dumped" flag set (exit code 128+signal, confirmed for all five).
The Windows path is implemented per the discussion's guidance but is
untested -- no Windows build available in this sandbox.

## Step 5 -- getting the data back out

- QETApp::checkCrashDump(), called from checkBackupFiles() only when
  there's no stale project file to recover this run (so the two
  prompts never both show, per the discussion), offers an unretrieved
  crash dump via DiagnosticsReportDialog and then deletes it regardless
  of the user's choice -- offered exactly once.
- A new "Aide > Enregistrer un rapport de diagnostic..." action
  (QETMainWindow) builds the same kind of report from the *current*
  session (QetLogger::buildDiagnosticsReport(): header + this session's
  log file) for a manual "attach this to a bug report" flow, not tied
  to a crash.
- Both go through QetLogger::redact() before ever reaching the user:
  the one redaction implemented is a literal replace of the home
  directory with "~", since an absolute path under it leaks the
  account name. The discussion's fancier "optionally redact project
  filenames too" isn't attempted -- reliably telling a project path
  apart from arbitrary log text is a much fuzzier problem than a
  literal prefix match.
- DiagnosticsReportDialog shows the full (already-redacted) content
  before saving, per the discussion: "the user is about to attach this
  to a public tracker."

Verified in a real GUI session (Xvfb): triggered a SIGSEGV, relaunched,
confirmed the crash-report dialog appears with the right header/content,
confirmed it does not reappear on a second relaunch, and confirmed the
manual "Save report" action produces a correctly-formatted report and
saves it to a chosen path.

Built clean, no new warnings.

## Build systems

Registered in both: cmake/qet_compilation_vars.cmake, and
qelectrotech.pro. The .pro needed explicit globs for the new
sources/logging/ui/ subfolder -- sources/logging/*.{h,cpp} was already
globbed, but unlike the other ui/ subfolders that one had no entry of
its own, so diagnosticsreportdialog.{h,cpp} would not have been built
under qmake.

sources/qetapp.h

@@ -271,6 +271,7 @@ class QETApp : public QObject
 		void openTitleBlockTemplateFiles(const QStringList &);
 		void configureQET();
 		void aboutQET();
+		void showDiagnosticsReport();
 		void receiveMessage(int instanceId, QByteArray message);
 	
 	private:
@@ -287,6 +288,7 @@ class QETApp : public QObject
 		void initSystemTray();
 		void buildSystemTrayMenu();
 		void checkBackupFiles();
+		void checkCrashDump();
 		void fetchWindowStats(
 			const QList<QETDiagramEditor *> &,
 			const QList<QETElementEditor *> &,