feat: browser capture button (Track 4)

- Add archivr-core/src/capture.rs: Source enum, all capture helpers,
  fail_run (replaces process::exit fail_archive_and_exit), and
  perform_capture() as the public entry point
- Slim archivr-cli/src/main.rs to 103 lines: thin adapter over
  archivr_core::capture::perform_capture
- Move all source-classification and tweet-entry tests from CLI to
  archivr-core/src/capture.rs (they test core logic, belong in core)
- Add POST /api/archives/:archive_id/captures route in archivr-server:
  validates non-empty locator (400), resolves archive (404), delegates
  to perform_capture, returns {run_uid, status}
- Add capture dialog to browser UI: dialog markup in index.html,
  showModal/submit/cancel/loading/error wiring in app.js, dialog
  styles in styles.css
- 77 tests pass, clean build (0 warnings)

crates/archivr-server/static/styles.css

@@ -496,3 +496,93 @@ select {
 .tag-filter-badge:hover {
   opacity: 0.85;
 }
+
+/* Capture dialog */
+.capture-dialog {
+  border: 2px solid var(--ink);
+  background: var(--paper);
+  padding: 0;
+  min-width: 360px;
+  max-width: 520px;
+  box-shadow: 4px 4px 0 var(--ink);
+}
+
+.capture-dialog::backdrop {
+  background: rgba(0, 0, 0, 0.45);
+}
+
+.capture-dialog-inner {
+  padding: 28px;
+}
+
+.capture-dialog-title {
+  margin: 0 0 16px;
+  font-size: 18px;
+  font-family: Georgia, "Times New Roman", serif;
+}
+
+.capture-label {
+  display: block;
+  font-size: 13px;
+  font-weight: 600;
+  margin-bottom: 6px;
+  color: var(--muted);
+  text-transform: uppercase;
+  letter-spacing: 0.04em;
+}
+
+.capture-input {
+  width: 100%;
+  height: 42px;
+  border: 2px solid var(--ink);
+  background: var(--paper-3);
+  color: var(--ink);
+  padding: 0 12px;
+  font-size: 15px;
+  margin-bottom: 10px;
+  outline-offset: 3px;
+}
+
+.capture-error {
+  color: var(--accent);
+  font-size: 13px;
+  margin-bottom: 10px;
+  min-height: 18px;
+}
+
+.capture-actions {
+  display: flex;
+  gap: 10px;
+  justify-content: flex-end;
+  margin-top: 16px;
+}
+
+.capture-cancel {
+  border: 1px solid var(--line);
+  background: none;
+  color: var(--ink);
+  padding: 8px 18px;
+  cursor: pointer;
+}
+
+.capture-cancel:hover {
+  background: var(--paper-2);
+}
+
+.capture-submit {
+  border: 0;
+  background: var(--ink);
+  color: var(--paper);
+  padding: 8px 20px;
+  cursor: pointer;
+  font-weight: 600;
+}
+
+.capture-submit:hover {
+  opacity: 0.85;
+}
+
+.capture-submit:disabled {
+  opacity: 0.45;
+  cursor: default;
+}