fix: polish explorer, error states and page alignment

- Explorer: 13px code, page-coloured dark editor, padded Markdown with a
  readable width, full repository ID on hover, sidebar footer kept clear
  of the floating Ko-fi button.
- Explorer errors render as a card with links home and to the FAQ; the
  empty action bar and Raw/Download buttons are hidden.
- Unknown error codes show the generic message instead of the raw
  ERRORS.<code> key.
- All app pages share one content column; New anonymization starts at
  the top instead of floating mid-screen.
- Status page: spaced action buttons, aligned status pill, no second
  Ko-fi embed under the support cards.
- Claim placeholders; consistent notebook option on new conferences.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

test/vue-ui.test.js

@@ -939,6 +939,18 @@ describe("Vue 3 UI", function () {
     expect(ui.errors).to.deep.equal([]);
   });
 
+  for (const [code, message] of [["repo_not_found", "The repository was not found"], ["untranslated_failure", "Unknown error, contact the admin."]]) {
+    it("shows the " + code + " explorer error in a card without file actions", async function () {
+      ui = await browser("/r/gone/", { "/api/repo/gone/options": { __status: 404, body: { error: code } } });
+      const card = ui.window.document.querySelector(".file-error-card");
+      expect(card.textContent).to.include("This content can").and.include(message);
+      expect(card.textContent).not.to.include("ERRORS.");
+      expect(card.querySelector('a[href="/"]')).to.not.equal(null);
+      expect(ui.window.document.querySelector(".status-bar").style.display).to.equal("none");
+      expect(ui.errors).to.deep.equal([]);
+    });
+  }
+
   it("renders hostile filenames as text and updates the explorer without losing the tree", async function () {
     const filename = '{{constructor.constructor("window.probe=1")()}}.txt';
     ui = await browser("/r/test/hello.txt", {