fix: reveal mode not working when already ON at page load

If reveal mode was saved as ON in settings, the page loaded with
revealMode=true and prevRevealMode=true. The checkRevealToggle
function only fires on transitions (off→on or on→off), so the
reveal interval was never started. Added initialization check
that starts the reveal interval immediately if revealMode is
already true at page load.

https://claude.ai/code/session_01KF4i7Ra7zCEDskxDBaNtcT

manifest.firefox.json

@@ -1,7 +1,7 @@
 {
   "manifest_version": 3,
   "name": "Silent Send",
-  "version": "0.9.10",
+  "version": "0.9.11",
   "description": "Intercepts personal info and substitutes it with user-defined replacements before sending to AI services.",
   "browser_specific_settings": {
     "gecko": {