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/keys.json

@@ -0,0 +1,23 @@
+{
+  "title": "Keyboard shortcuts",
+  "contextMenu": "Open the context menu",
+  "undo": "Undo",
+  "redo": "Redo",
+  "playPause": "Play / pause",
+  "fullscreen": "Fullscreen",
+  "seek": "Seek 10 seconds",
+  "seek5": "Seek 5 seconds",
+  "fineSeek": "Fine seek",
+  "episode": "Previous / next episode",
+  "video": "Previous / next video",
+  "volume": "Volume up / down",
+  "mute": "Mute",
+  "audio": "Cycle audio track",
+  "subtitle": "Cycle subtitle",
+  "help": "Show this help",
+  "info": "Media info",
+  "stats": "Stats for nerds",
+  "back": "Stop & back to feed",
+  "browseBack": "Back one level",
+  "closePlayer": "Close the player"
+}

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",

frontend/src/i18n/locales/hu/keys.json

@@ -0,0 +1,23 @@
+{
+  "title": "Billentyűparancsok",
+  "contextMenu": "Helyi menü megnyitása",
+  "undo": "Visszavonás",
+  "redo": "Újra",
+  "playPause": "Lejátszás / szünet",
+  "fullscreen": "Teljes képernyő",
+  "seek": "Tekerés 10 másodperc",
+  "seek5": "Tekerés 5 másodperc",
+  "fineSeek": "Finom léptetés",
+  "episode": "Előző / következő rész",
+  "video": "Előző / következő videó",
+  "volume": "Hangerő fel / le",
+  "mute": "Némítás",
+  "audio": "Hangsáv váltása",
+  "subtitle": "Felirat váltása",
+  "help": "Súgó megjelenítése",
+  "info": "Média infó",
+  "stats": "Statisztika (nerdöknek)",
+  "back": "Leállítás és vissza a feedre",
+  "browseBack": "Vissza egy szintet",
+  "closePlayer": "Lejátszó bezárása"
+}

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

@@ -186,21 +186,6 @@
     "stop": "Leállítás",
     "help": "Billentyűparancsok",
     "infoBtn": "Média infó",
-    "keys": {
-      "playPause": "Lejátszás / szünet",
-      "seek": "Tekerés 10 másodperc",
-      "episode": "Előző / következő rész",
-      "volume": "Hangerő fel / le",
-      "mute": "Némítás",
-      "fullscreen": "Teljes képernyő",
-      "info": "Média infó",
-      "back": "Leállítás és vissza a feedre",
-      "help": "Súgó megjelenítése",
-      "audio": "Hangsáv váltása",
-      "subtitle": "Felirat váltása",
-      "fineSeek": "Finom léptetés",
-      "stats": "Statisztika (nerdöknek)"
-    },
     "upNext": {
       "title": "Következik",
       "playNow": "Lejátszás most",