feat: v0.3.0 — multiple entries per field, README setup guide

Identity fields now support multiple entries per type:
- Add unlimited names (first, last, middle, nickname), emails,
  usernames, hostnames, and phone numbers per profile
- "+ Add" button on each section, "x" to remove rows
- Names have a type selector (1st/Last/Mid/Nick)

README now includes:
- First-time setup walkthrough (step by step)
- Icon color legend (gray/black/blue/red)
- Keyboard shortcuts table
- Note that extension does nothing until configured

Also bumps version to 0.3.0 for Firefox re-signing.

https://claude.ai/code/session_01Dvgwe7XMoSxnWXkih8p1Cw

src/options/options.html

@@ -136,7 +136,7 @@
     </section>
 
     <footer>
-      <p>Silent Send v0.2.0</p>
+      <p>Silent Send v0.3.0</p>
     </footer>
   </div>