Make the YouTube account reachable, and applied without a restart

The cookie login was not missing, it was buried: Downloads tab -> Custom
Options -> the "Login with Cookies" tab, where Downloads is one tab of five.
The three features that depend on it -- the Feed's account mode, Search and
Browse -- are other tabs, with no route to it. show_cookie_login_dialog(),
which opens that dialog on the right tab, had no caller anywhere in the
codebase; it does now.

An account button sits in the corner of the tab bar, visible from every tab,
saying whether cookies are in use and where they came from. The Feed offers
the same thing next to its account mode rather than greying the option out and
leaving it at that, and the disabled entry now says why it is disabled.

Applying cookies used to require a restart before playback saw them:
_build_ytdl_raw_options was read once, when the player widget was built. A
cookiesChanged signal now reaches the account button, the Feed and the live
mpv handle, and the player reloads at its current position -- ytdl_hook
consults the option when it resolves a URL, so anything already playing keeps
the streams it resolved with. YtdlpClient needed no change: it re-reads config
per call.

Two bugs in the same function: the options are joined with commas and were
never escaped, so a cookie path or a proxy URL containing one silently ended
the option and started a bogus one; and geo_proxy_url was honoured by the
downloader but never passed to the player, so a geo-restricted video would
download and then refuse to play.

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

ytsage/languages/en.json

@@ -757,6 +757,16 @@
     "refreshed_with_failures": "{count} videos — {failed} channel(s) could not be reached",
     "account_needs_refresh": "Press Refresh to load your account feed",
     "empty_no_subscriptions": "No subscriptions yet.\n\nFind channels in Search or Browse, then use Subscribe to follow them — their new uploads will appear here.",
-    "empty_not_refreshed": "Nothing here yet.\n\nPress Refresh to fetch the latest uploads from your subscriptions."
+    "empty_not_refreshed": "Nothing here yet.\n\nPress Refresh to fetch the latest uploads from your subscriptions.",
+    "sign_in": "Sign in with cookies…",
+    "sign_in_tooltip": "Use your YouTube account via browser cookies, which enables the account feed",
+    "account_requires_cookies": "Requires signing in with cookies"
+  },
+  "account": {
+    "signed_out": "Not signed in",
+    "signed_out_tooltip": "SageTube is not using a YouTube account. Sign in with browser cookies to use your subscription feed and reach age-restricted or members-only videos.",
+    "signed_in": "Account: {source}",
+    "signed_in_tooltip": "Using cookies from {source}. Click to change or clear them.",
+    "source_file": "cookie file"
   }
 }