feat(gmail,update): finish the OAuth2 flow, and tell admins about new releases

Gmail now connects. Authorisation-code flow with PKCE, the verifier kept
server-side rather than in a cookie — the callback is a top-level
navigation from Google, and a SameSite=Lax cookie is the kind of thing
that works until a browser tightens its defaults. State is checked, the
attempt expires after ten minutes, and the spent verifier is deleted
rather than left at rest.

Scope is gmail.readonly and the sync is one-way on purpose. Unstarring a
message does not delete the task; completing the task does not touch the
mailbox. Two-way would mean deciding what happens when both sides
changed, and the only honest answer is a sync model far larger than the
feature. The query carries newer_than:30d so connecting an account with
a decade of starred mail does not create a decade of tasks.

The update check is off unless the operator turns it on, asks GitHub at
most every six hours, and sends nothing about the instance — no
identifier, no version, no counts. A development build never reports an
update: somebody running their own tree is ahead of every release, and
telling them otherwise trains them to ignore the notice. Only
administrators see it, because only they can act on it.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

internal/config/config.go

@@ -28,6 +28,16 @@ type Config struct {
 
 	SMTP SMTP
 
+	// Gmail is the OAuth client the operator registered in Google Cloud. It
+	// belongs to the instance: one registration, any number of users.
+	GmailClientID     string
+	GmailClientSecret string
+
+	// UpdateCheck asks GitHub whether a newer release exists. Off unless asked
+	// for: a self-hosted app that reaches out without being told to has broken
+	// the deal its operator made by self-hosting.
+	UpdateCheck bool
+
 	// TrashRetention is how long a soft-deleted task or project stays recoverable.
 	TrashRetention time.Duration
 
@@ -57,6 +67,10 @@ func Load() (*Config, error) {
 		DataDir:        env("VERDANDE_DATA_DIR", "/data"),
 		TrashRetention: 30 * 24 * time.Hour,
 		Dev:            envBool("VERDANDE_DEV", false),
+
+		GmailClientID:     env("VERDANDE_GMAIL_CLIENT_ID", ""),
+		GmailClientSecret: env("VERDANDE_GMAIL_CLIENT_SECRET", ""),
+		UpdateCheck:       envBool("VERDANDE_UPDATE_CHECK", false),
 	}
 
 	var err error
@@ -98,6 +112,11 @@ func (c *Config) DBPath() string     { return filepath.Join(c.DataDir, "verdande
 func (c *Config) FilesDir() string   { return filepath.Join(c.DataDir, "files") }
 func (c *Config) BackupsDir() string { return filepath.Join(c.DataDir, "backups") }
 
+// GmailRedirectURL has to match what was registered in Google Cloud exactly,
+// including the scheme — which is why it is derived from BaseURL rather than
+// configured separately and left to drift out of step with it.
+func (c *Config) GmailRedirectURL() string { return c.BaseURL + "/oauth/gmail/callback" }
+
 func env(key, def string) string {
 	if v, ok := os.LookupEnv(key); ok && v != "" {
 		return v