feat: integrate Modal Closer as injected browser script

Port the modalcloser abx-plugin as a SingleFile --browser-script rather
than a spawned Puppeteer daemon. No external extension download required.

Core behavior (singlefile.rs):
- MODAL_CLOSER_DIALOG_OVERRIDES: main-world <script> bridge (best-effort;
  blocked by strict script-src CSP) that overrides window.alert/confirm/
  prompt/print, nulls window.onbeforeunload, traps its setter, and wraps
  window.addEventListener to no-op 'beforeunload' registrations.
- MODAL_CLOSER_POLLING_SETUP: defines _archivr_mc_run() which runs two
  passes on every tick (immediate first run, then setInterval every 500 ms
  matching MODALCLOSER_POLL_INTERVAL default):
    Pass 1 (best-effort, CSP-sensitive): main-world <script> bridge calls
    Bootstrap/jQuery/jQuery UI/SweetAlert teardown APIs.
    Pass 2 (always, CSP-immune): isolated-world DOM mutations — Escape-key
    dispatch (Radix/Headless UI/Angular Material), backdrop clicks, full
    CSS selector hiding for 40+ named consent/overlay vendors, body
    scroll-lock reset. Direct DOM mutations need no inline script execution.
- resolve_modal_closer_config(): reads ARCHIVR_MODAL_CLOSER env var
  (default true); no external resource required.
- modal_closer_enabled: Option<bool> in CaptureConfig so Default::default()
  yields None (follow env var) rather than false.

Schema (database.rs):
- modal_closer_enabled column in auth DB instance_settings (default 1).
- Idempotent ALTER TABLE migration for existing databases.
- get/update_instance_settings updated (SELECT col 6, UPDATE param ?7).

HTTP (routes.rs):
- modal_closer_enabled in CaptureBody and UpdateInstanceSettingsBody.
- Per-capture body overrides global setting which overrides env var.

Frontend:
- SettingsView ExtensionsTab: third ext-card for Modal & Dialog Closer.
- CaptureDialog: per-capture toggle in Advanced Options, state initialized
  from server default, included in submitCapture payload.
- api.js: submitCapture forwards modal_closer_enabled to capture body.

Behavior vs original modalcloser daemon:
- Functionally identical on non-strict-CSP pages (the large majority).
- Gap: <script> bridge is subject to page CSP; page.evaluate() is not.
  On strict-CSP pages framework teardown and dialog overrides are no-ops;
  CSS selector hiding and Escape dispatch (Pass 2) always work.
- Gap: alert/confirm/prompt overridden to instant no-op vs CDP timed
  dialog.accept() with 1250 ms delay; irrelevant for archival in practice.

crates/archivr-core/src/capture.rs

@@ -85,6 +85,8 @@ pub struct CaptureConfig {
     pub cookie_ext_enabled: Option<bool>,
     /// Apply Mozilla Readability to distil the page to article content before archiving.
     pub reader_mode: bool,
+    /// Override for modal-closer browser-script behavior during WebPage captures.
+    pub modal_closer_enabled: Option<bool>,
 }
 
 /// Resolves which cookies apply to `url` by evaluating all rules in ordinal order.
@@ -1036,7 +1038,7 @@ pub fn perform_capture(
 
     // Source: web page — archive as a self-contained HTML snapshot via single-file-cli
     if source == Source::WebPage {
-        match downloader::singlefile::save(locator, store_path, &timestamp, &cookies, config.ublock_enabled, config.cookie_ext_enabled, config.reader_mode) {
+        match downloader::singlefile::save(locator, store_path, &timestamp, &cookies, config.ublock_enabled, config.cookie_ext_enabled, config.reader_mode, config.modal_closer_enabled) {
             Ok(result) => {
                 let file_extension = ".html".to_string();
                 let temp_html = store_path