feat: add profiles, public views, settings, and code quality fixes

Phase 3 — Profiles & Public Views:
- Public profile page (/u/{username}) with OG meta tags
- User settings page (display name, bio, visibility, default privacy)
- Avatar upload with image processing
- Password change from settings (verifies current password)
- Home page shows public fave feed for logged-in users
- Must-reset-password guard redirects to /reset-password
- Profile visibility: public (full) or limited (username only)

Code quality improvements from /simplify review:
- Fix signup request persistence bug (was silently discarding data)
- Fix health check to use configured listen address, not hardcoded :8080
- Add rate limiter cleanup goroutine (was leaking memory)
- Extract shared helpers: ClearSessionCookie, IsSecureRequest, scanTags,
  scanUserFrom (scanner interface), SignupRequestStore
- Replace hand-rolled joinPlaceholders with strings.Join
- Remove dead _method hidden field, redundant devMode field
- Simplify rate-limited route registration (remove double-mux)
- Log previously-swallowed errors (session delete, image delete)
- Stop leaking internal error messages to users in image upload

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

web/static/css/style.css

@@ -88,6 +88,23 @@
     vertical-align: middle;
 }
 
+/* Profile header */
+.profile-header {
+    display: flex;
+    align-items: center;
+    gap: 1.5rem;
+    margin-bottom: 1rem;
+}
+
+/* Avatar */
+.avatar-large {
+    width: 120px;
+    height: 120px;
+    border-radius: 50%;
+    object-fit: cover;
+    flex-shrink: 0;
+}
+
 /* Tag chips */
 .tag-chip {
     display: inline-block;