feat: uBlock Origin Lite integration for ad-blocking during WebPage captures

- singlefile.rs: when ARCHIVR_UBLOCK=true and ARCHIVR_UBLOCK_EXT is set,
  archivr owns Chrome's lifecycle (--headless=new, --remote-debugging-port,
  --load-extension); single-file connects via --browser-server instead of
  launching its own Chrome. Falls back to old behaviour with ublock_skipped=true
  when the ext path is missing or invalid.
- capture.rs: thread ublock_skipped through CaptureResult
- database.rs: add notes_json TEXT column to capture_jobs (DDL + idempotent
  ALTER TABLE migration); update_capture_job_status gains notes_json param
- archive.rs: expose notes_json in CaptureJobSummary
- routes.rs: store {"ublock_skipped":true} in notes_json on completed captures
- ToastStack.jsx: warning toast variant (toast--warning) with Details expander
  and Ignore button
- CaptureDialog.jsx: fire warning toast when poll result has ublock_skipped
- App.jsx: sessionStorage-backed Ignore suppression for ublock warnings
- styles.css: .toast--warning (amber left border) + .toast-warning-detail
- flake.nix: ublockLite derivation fetches uBOLite_2026.705.2152.chromium.zip
  (pinned SHA256) from uBlockOrigin/uBOL-home; sets ARCHIVR_UBLOCK_EXT in both
  archivr and archivr-server wrappers

Env vars:
  ARCHIVR_UBLOCK=true (default) — enable uBlock during WebPage captures
  ARCHIVR_UBLOCK_EXT — path to unpacked uBOL extension dir (set by Nix)

frontend/src/styles.css

@@ -812,6 +812,7 @@ select {
   to   { opacity: 1; transform: translateY(0) scale(1); }
 }
 .toast--error { border-left: 3px solid var(--accent); }
+.toast--warning { border-left: 3px solid #e8a000; }
 
 .toast-top {
   display: flex;
@@ -892,6 +893,18 @@ select {
   max-height: 200px;
   overflow-y: auto;
 }
+.toast-warning-detail {
+  margin: 0;
+  padding: 10px 14px 12px;
+  font-size: 12px;
+  line-height: 1.55;
+  color: var(--muted);
+  background: var(--paper-2);
+  border-top: 1px solid var(--line);
+  white-space: pre-wrap;
+  word-break: break-word;
+}
+.toast-ignore-btn { color: var(--muted); }
 
 /* ── Utility ─────────────────────────────────────────────────────────────── */
 .muted { color: var(--muted); }