fix(app): stop trusting X-Forwarded-For, and pin what the audit found

ProxyFix defaults x_for to 1, so ProxyFix(app.wsgi_app, x_proto=1) never
disabled it: request.remote_addr and the access log were forgeable by any
client that reached the app directly. It is x_for=0 now, asserted rather
than assumed.

A mutation audit over the change set reverted 196 deliberate behaviours
and found 47 that no test noticed. This closes the ones that carry damage:

- apod_background lost its key check, its transport guard, its status
  guard and its media-type check without a single test failing. Each one
  turns a slow or unhappy NASA into a 500 on every page.
- Untrusted values reached innerHTML through window.I18N, which the
  translation backend writes, and the modal's click handlers stacked so a
  later click opened an earlier popup's URL.
- The sync tool could ask for HTML instead of text, translate from "auto"
  instead of English, run without a timeout, store an empty translation
  that marks the string done for good, abandon 28 languages because one
  could not be written, and report success after reaching nothing.
- Neither the lint target, the CI jobs, the vendored RTL stylesheet, the
  documented environment keys, nor any of the four hardenings in
  scripts/run-e2e.sh was observed by anything.

Three of the new tests passed for the wrong reason on their first cut —
a mock that answered None whether or not the guard existed, a
raise_for_status that was never called, a string that stayed in the file
after the mutation. The audit found those too; all 24 reverts now fail.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

app/app.py

@@ -70,7 +70,7 @@ app = Flask(__name__)
 
 app.jinja_options = {**app.jinja_options, "autoescape": True}
 
-app.wsgi_app = ProxyFix(app.wsgi_app, x_proto=1)
+app.wsgi_app = ProxyFix(app.wsgi_app, x_for=0, x_proto=1)
 
 
 def trusted_hosts(raw):

app/cypress/e2e/injection.spec.js

@@ -72,6 +72,21 @@ describe('Untrusted content in the modal', () => {
       cy.window().should('not.have.property', '__xss');
     });
 
+    it('keeps a relative link', () => {
+      open({ warning: 'See [the notes](/#anchor)' });
+
+      cy.get('#dynamicModalWarningText')
+        .find('a')
+        .should('have.attr', 'href', '/#anchor');
+    });
+
+    it('keeps the text of a link it strips', () => {
+      open({ warning: '[read this](javascript:window.__xss=1)' });
+
+      cy.get('#dynamicModalWarningText').should('contain.text', 'read this');
+      cy.window().should('not.have.property', '__xss');
+    });
+
     it('keeps ordinary markdown', () => {
       open({ warning: 'See [Matrix](https://matrix.org/) and **mind** this' });
 
@@ -83,6 +98,40 @@ describe('Untrusted content in the modal', () => {
   });
 
   describe('values interpolated outside markdown', () => {
+    it('does not treat an interface string as markup', () => {
+      cy.window().then(win => {
+        win.I18N.Open = '<img src=x onerror="window.__xss = true">';
+      });
+      open({
+        alternatives: [
+          { name: 'Alt', identifier: 'A', icon: { class: 'fa-alt' } },
+        ],
+      });
+
+      cy.get('#dynamicAlternativesList').find('img').should('not.exist');
+      cy.get('#dynamicAlternativesList').should('contain.text', 'onerror');
+      cy.window().should('not.have.property', '__xss');
+    });
+
+    it('falls back to the English source when a string is missing', () => {
+      cy.window().then(win => {
+        delete win.I18N;
+      });
+      open({
+        alternatives: [
+          { name: 'Alt', identifier: 'A', icon: { class: 'fa-alt' } },
+        ],
+      });
+
+      cy.get('#dynamicAlternativesList button').should('have.text', 'Open');
+    });
+
+    it('renders no placeholder for a missing name', () => {
+      open({ name: undefined });
+
+      cy.get('#dynamicModalLabel').should('not.contain.text', 'undefined');
+    });
+
     it('does not treat the name or the icon class as markup', () => {
       open({
         name: '<img src=x onerror="window.__xss = true">',
@@ -123,6 +172,12 @@ describe('Untrusted content in the modal', () => {
       );
     });
 
+    it('keeps a URL that carries surrounding whitespace', () => {
+      open({ url: ' https://example.com ', description: 'Good' });
+
+      cy.get('#dynamicModalLinkHref').should('have.attr', 'href');
+    });
+
     it('keeps a mailto URL', () => {
       open({ url: 'mailto:kevin@veen.world', description: 'Write' });
 
@@ -156,6 +211,17 @@ describe('Untrusted content in the modal', () => {
         expect($anchor[0].onclick, 'stale click handler').to.equal(null);
       });
     });
+
+    it('opens the current popup URL, not an earlier one', () => {
+      open({ url: 'https://a.test/', description: 'A', iframe: true });
+      open({ url: 'https://b.test/', description: 'B', iframe: true });
+
+      cy.get('#dynamicModalLinkHref').click();
+
+      cy.get('#main')
+        .find('iframe', { timeout: 4000 })
+        .should('have.attr', 'src', 'https://b.test/');
+    });
   });
 });