ext: context-menus.ts — link/media menu items + grab-tab command

docs/05 §3: "Download with Velox" on a link (linkUrl) or a media element
(srcUrl) hands that one URL to download.add with the page as referrer; a
configurable command (Ctrl+Shift+U) does the same for the active tab. These
are explicit user requests, so they skip shouldCapture and the fail-open
path — a failure is surfaced to the user via notifications instead.

The page/selection "Download all links…" item waits on the content-script
link harvester (build-order step 7) and will be added there.

manifest: commands.velox-grab-current-tab. 9 tests. 110 total, lint clean.

Co-Authored-By: Claude Sonnet 5 <[email protected]>
Claude-Session: https://claude.ai/code/session_012Y9RU58hD1BuwP82DySUHk

extension/manifest.json

@@ -30,5 +30,12 @@
     "nativeMessaging"
   ],
 
-  "host_permissions": ["<all_urls>"]
+  "host_permissions": ["<all_urls>"],
+
+  "commands": {
+    "velox-grab-current-tab": {
+      "suggested_key": { "default": "Ctrl+Shift+U" },
+      "description": "Send the current tab's URL to Velox"
+    }
+  }
 }