chore: sync all versions to 2.0.0 + fix sign script

Set Chrome, Firefox, and package.json all to version 2.0.0
(major bump reflecting encrypted storage, document scanning,
org/team features, sync improvements).

Sign script rewritten:
- Tries current version first instead of always bumping
- Only bumps on "version already exists" errors
- Handles rate limiting by parsing throttle duration from error
  and waiting the exact time (not blindly retrying)
- Only updates source files on successful signing (not before)
- Reduced max attempts to 5 (with proper backoff, shouldn't need more)
- Commits version bump only after successful sign

https://claude.ai/code/session_01SWSwDfMVij53bCTNSCLMwn

manifest.firefox.json

@@ -1,7 +1,7 @@
 {
   "manifest_version": 3,
   "name": "Silent Send",
-  "version": "0.3.1",
+  "version": "2.0.0",
   "description": "Intercepts personal info and substitutes it with user-defined replacements before sending to AI services.",
   "browser_specific_settings": {
     "gecko": {