feat: mobile logo, expandable history, fix session resume

- Show Codeman logo on mobile as compact home button (was hidden)
- Add "Show More" button for history sessions (initial 4, expand all)
- Deduplicate by projectKey instead of workingDir (lossy decode fix)
- Fix project key decoding: handle '_' encoded as '-' with look-ahead
- Pre-validate resumeSessionId before passing to Claude CLI
- Apply content validation to all session files regardless of size

src/web/public/mobile.css

@@ -301,9 +301,15 @@ html.mobile-init .file-browser-panel {
    Phone Breakpoint (<430px)
    ============================================================================ */
 @media (max-width: 430px) {
-  /* Hide header brand on phones */
+  /* Compact header brand on phones — acts as home button */
   .header-brand {
-    display: none;
+    padding-right: 0.25rem;
+    margin-right: 0.2rem;
+    border-right: none;
+  }
+
+  .header-brand .logo {
+    font-size: 0.7rem;
   }
 
   /* Font controls - compact on phones, visibility controlled by JS */