fix: security hardening, blur geometry, and performance audit fixes (v0.6.2)

Security: cache dirs now 0o700 via DirBuilder::mode(), blur config
validated (finite + clamp 0–200), TOCTOU socket pre-check removed.

Quality: GPU blur geometry fixed (texture shifted instead of stretched),
is_valid_username hardened, is_valid_gtk_theme extracted as testable fn,
save_last_session error handling consistent with save_last_user.

Performance: blurred texture cached across monitors (1x GPU renderpass
instead of N), FingerprintProbe device proxy cached in GreeterState with
generation counter to prevent race condition on fast user-switch.

Clippy: all 7 warnings resolved (collapsible if-let, redundant closure,
manual_range_contains, too_many_arguments suppressed for GTK widget fns).

Tests: 109 → 118 (GTK theme validation, Unicode usernames, cache dir
permissions, unwritable dir handling, blur config edge cases).

src/i18n.rs

@@ -19,8 +19,6 @@ pub struct Strings {
     pub no_session_selected: &'static str,
     pub greetd_sock_not_set: &'static str,
     pub greetd_sock_not_absolute: &'static str,
-    pub greetd_sock_not_socket: &'static str,
-    pub greetd_sock_unreachable: &'static str,
     pub auth_failed: &'static str,
     pub wrong_password: &'static str,
     pub fingerprint_prompt: &'static str,
@@ -43,8 +41,6 @@ const STRINGS_DE: Strings = Strings {
     no_session_selected: "Keine Session ausgewählt",
     greetd_sock_not_set: "GREETD_SOCK nicht gesetzt",
     greetd_sock_not_absolute: "GREETD_SOCK ist kein absoluter Pfad",
-    greetd_sock_not_socket: "GREETD_SOCK zeigt nicht auf einen Socket",
-    greetd_sock_unreachable: "GREETD_SOCK nicht erreichbar",
     auth_failed: "Authentifizierung fehlgeschlagen",
     wrong_password: "Falsches Passwort",
     fingerprint_prompt: "Fingerabdruck auflegen oder Passwort eingeben",
@@ -65,8 +61,6 @@ const STRINGS_EN: Strings = Strings {
     no_session_selected: "No session selected",
     greetd_sock_not_set: "GREETD_SOCK not set",
     greetd_sock_not_absolute: "GREETD_SOCK is not an absolute path",
-    greetd_sock_not_socket: "GREETD_SOCK does not point to a socket",
-    greetd_sock_unreachable: "GREETD_SOCK unreachable",
     auth_failed: "Authentication failed",
     wrong_password: "Wrong password",
     fingerprint_prompt: "Place finger on reader or enter password",