fix(secure_storage): fix silent error swallowing in GError handling

[aki1770-del]

Problem

keyring.h uses the following pattern in storeToKeyring and
readFromKeyring:

std::unique_ptr<GError> err = nullptr;
GError* errPtr = err.get();                 // errPtr == nullptr
secret_password_storev_sync(..., &errPtr);  // libsecret writes into errPtr
if (err) { ... }                            // always false — err never updated

errPtr = err.get() copies the null pointer value into a local variable.
GLib writes a newly-allocated GError* into *(&errPtr) — the local
variable — not into the unique_ptr. The unique_ptr is never updated.

Consequences:

• libsecret write/read errors are silently swallowed
• The function returns true (success) when the underlying operation failed
• Any GError* written into errPtr by libsecret is leaked

Fixes #230

Changes

File: plugins/secure_storage/keyring.h
Methods: storeToKeyring, readFromKeyring

Replace std::unique_ptr<GError> err + GError* errPtr with a plain
GError* err, pass &err directly to the libsecret API, copy the
message string before g_clear_error, then throw.

This brings keyring.h into line with the GError* pattern already
used in this repo — flatpak_shim.cc (two sites) and video_player.cc
(one site). No new pattern is introduced.

Fix Pattern

// Before (broken — err always null, errPtr leaked):
std::unique_ptr<GError> err = nullptr;
GError* errPtr = err.get();
secret_password_storev_sync(..., &errPtr);
if (err) { throw std::runtime_error(err->message); }

// After (correct — matches existing repo pattern):
GError* err = nullptr;
secret_password_storev_sync(..., &err);
if (err) {
  std::string msg = err->message;  // copy before g_clear_error frees the struct
  g_clear_error(&err);
  throw std::runtime_error(msg);
}

Verification

• Success path: err remains null — behavior unchanged from the caller's
  perspective
• Error path: err is correctly set by libsecret; message is copied before
  g_clear_error frees the struct; g_clear_error resolves the memory leak;
  exception is thrown with the correct message
• Callers catching std::runtime_error are unaffected — exception contract
  is unchanged
• No includes added or removed

Signed-off-by: aki1770-del aki1770@gmail.com

[aki1770-del]

Please don't merge this one — I've moved it to draft.

The change makes a dead throw live. Today err in keyring.h is never reassigned (the callee fills a copy), so both if (err) are false and both throws are unreachable. Fixing that makes them reachable, and there is nothing to catch them: messages.cc has no catch and carries #undef _HAS_EXCEPTIONS at :7, the Flutter C++ client wrapper has none, and the seven in shell/ are on other paths. It reaches the engine's C callback, so std::terminate.

A keyring failure would go from returning a wrong answer to ending the homescreen process. On a head unit that is worse than the bug.

The silent success is also not in the file this PR touches: messages.cc:69/79/83 call api->write(), which returns void, then report true unconditionally. That is Pigeon-generated.

Nothing needed from you. Reworking it against #230.