fix: PPI → PII across entire codebase + add Ko-fi support section

PII (Personally Identifiable Information) is the correct standard term.
Renamed all instances — comments, UI labels, variable names, function
names (autoDetectPPI → autoDetectPII, scanInputForPPI → scanInputForPII,
ppiWarnings → piiWarnings), CSS comments, README, and options page.

Added Ko-fi donation section to README and Options footer. Tone: no
obligation, no warranty, no influence on updates — just appreciation.

https://claude.ai/code/session_01KF4i7Ra7zCEDskxDBaNtcT

src/lib/storage.js

@@ -23,7 +23,7 @@ const KEYS = {
   SETTINGS: 'ss_settings',
 };
 
-// Keys that contain sensitive PPI and should be encrypted at rest
+// Keys that contain sensitive PII and should be encrypted at rest
 // All user data keys are encrypted at rest — settings included since
 // custom domains and configuration can reveal what services the user
 // accesses. Only ss_sync_encryption (salt, verification blob) and