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)

crates/archivr-core/src/capture.rs

@@ -37,6 +37,9 @@ pub enum Source {
 pub struct CaptureResult {
     pub run_uid: String,
     pub status: String,
+    /// `true` when uBlock was requested but the extension path was not found.
+    /// The capture succeeded without ad-blocking; the UI should warn the user.
+    pub ublock_skipped: bool,
 }
 
 #[derive(Debug, Clone, Default)]
@@ -991,6 +994,7 @@ pub fn perform_capture(
                 return Ok(CaptureResult {
                     run_uid: run.run_uid.clone(),
                     status: "completed".to_string(),
+                    ublock_skipped: false,
                 });
             }
             Err(e) => {
@@ -1133,6 +1137,7 @@ pub fn perform_capture(
                 return Ok(CaptureResult {
                     run_uid: run.run_uid.clone(),
                     status: "completed".to_string(),
+                    ublock_skipped: result.ublock_skipped,
                 });
             }
             Err(e) => {
@@ -1187,6 +1192,7 @@ pub fn perform_capture(
                 return Ok(CaptureResult {
                     run_uid: run.run_uid.clone(),
                     status: "completed".to_string(),
+                    ublock_skipped: false,
                 });
             }
             Err(e) => {
@@ -1342,6 +1348,7 @@ pub fn perform_capture(
     Ok(CaptureResult {
         run_uid: run.run_uid.clone(),
         status: "completed".to_string(),
+        ublock_skipped: false,
     })
 }