feat: Reader mode via Mozilla Readability.js

Adds an opt-in 'Reader mode' advanced option to the capture dialog.
When enabled, Readability.js is injected as a browser script during
SingleFile capture; it fires on single-file-on-before-capture-start,
replaces the page body with the distilled article content, injects a
clean typographic stylesheet, and adds a header with title/byline/site.
Falls back silently if Readability fails (e.g. non-article pages).

- vendor/readability/Readability.js  Apache 2.0, Mozilla, v0.6.0
- singlefile.rs: embed READABILITY_JS + READER_MODE_WRAPPER_JS via
  include_str!; write both to temp dir when reader_mode is true;
  base_single_file_cmd now accepts &[&Path] for multiple --browser-script
- capture.rs: CaptureConfig.reader_mode: bool
- routes.rs: CaptureBody.reader_mode: Option<bool> (defaults false)
- api.js: submitCapture passes reader_mode in payload
- CaptureDialog.jsx: Reader mode toggle in Advanced options (off by default)

frontend/src/api.js

@@ -98,9 +98,10 @@ export async function fetchTags(archiveId) {
 export async function submitCapture(archiveId, locator, quality = null, extensions = null) {
   const payload = { locator }
   if (quality && quality !== 'best') payload.quality = quality
-  // extensions: { ublock_enabled?: bool } — per-capture overrides
-  if (extensions && typeof extensions.ublock_enabled === 'boolean') {
-    payload.ublock_enabled = extensions.ublock_enabled
+  // extensions: { ublock_enabled?: bool, reader_mode?: bool } — per-capture overrides
+  if (extensions) {
+    if (typeof extensions.ublock_enabled === 'boolean') payload.ublock_enabled = extensions.ublock_enabled
+    if (typeof extensions.reader_mode === 'boolean') payload.reader_mode = extensions.reader_mode
   }
   const res = await fetch(`/api/archives/${archiveId}/captures`, {
     method: "POST",