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/content/content.css

@@ -35,7 +35,7 @@
   border-radius: 2px;
 }
 
-/* Auto-detect PPI warning banner */
+/* Auto-detect PII warning banner */
 .ss-autodetect-warning {
   position: fixed;
   top: 16px;
@@ -136,7 +136,7 @@
   font-style: italic;
 }
 
-/* Pre-send PPI warning (spellcheck-style, appears while typing) */
+/* Pre-send PII warning (spellcheck-style, appears while typing) */
 .ss-presend-warning {
   position: fixed;
   top: 16px;