feat(e10): put every keyboard shortcut in one table

Six surfaces each owned a private keydown listener with its own key
literals and its own idea of "am I typing?", and they had drifted:
PlexPlayer matched `case "a"` without checking modifiers, so Ctrl+A
(select all) cycled the audio track, and its typing guard missed
<select> and contentEditable where the other two surfaces covered both.

lib/keymap.ts is a TABLE, not a dispatcher. Each surface keeps its own
listener and closures - PlexPlayer's refs are not worth re-plumbing -
but the chords, the modifier rules and the typing guard live in one
place, and dispatch() binds a scope's rows to handlers through a mapped
type, so a row without a handler (or a handler without a row) is a tsc
error rather than a comment someone forgets.

Matching is now exact on modifiers, which is what kills the Ctrl+A bug
and lets Shift+arrow be its own row beside the bare arrow. Escape opts
out of the typing guard (allowInInput) so it stays the way out of a
player's own subtitle-offset field.

The player's shortcut sheet renders helpLines() instead of a hand-copied
array beside the handler - the two used to be edited separately, which
is how a sheet ends up documenting a key that no longer exists. Labels
move to the new `keys` i18n namespace (EN + HU), and a test asserts
every row is named in both.

frontend/src/i18n/locales/en/plex.json

@@ -186,21 +186,6 @@
     "stop": "Stop",
     "help": "Keyboard shortcuts",
     "infoBtn": "Media info",
-    "keys": {
-      "playPause": "Play / pause",
-      "seek": "Seek 10 seconds",
-      "episode": "Previous / next episode",
-      "volume": "Volume up / down",
-      "mute": "Mute",
-      "fullscreen": "Fullscreen",
-      "info": "Media info",
-      "back": "Stop & back to feed",
-      "help": "Show this help",
-      "audio": "Cycle audio track",
-      "subtitle": "Cycle subtitle",
-      "fineSeek": "Fine seek",
-      "stats": "Stats for nerds"
-    },
     "upNext": {
       "title": "Up next",
       "playNow": "Play now",