Fix slash-command suggestions that insert commands the processor rejects (#1354)

The autocomplete panel is the only in-app surface for discovering slash
commands, but several suggestions do not match what CommandProcessor
accepts, so tapping them inserts a command that returns "unknown
command":

- CommandInfo suggests /dm, /favorite, /unfavorite, but the processor
  only handles /m, /msg, /fav, /unfav. Aliases are aligned to the
  accepted spellings (msg, fav, unfav).
- Favorites are suggested only in geohash contexts (isGeoPublic ||
  isGeoDM) — exactly where the processor rejects them ("favorites are
  only for mesh peers"). The gating is inverted so they appear in mesh,
  where they work.

Also, small related fixes to the discovery surface:
- /help is now handled (the ChatViewModel command docstring already
  claimed it existed); it prints a local system line listing the valid
  commands, and the unknown-command error points at it.
- The suggestion panel keeps the matched command's usage row (e.g.
  "/msg <nickname>") visible while arguments are typed, instead of
  vanishing at the first space; in that mode the row is informational
  and no longer overwrites the draft on tap.

New string is added source-language (en) only. The CommandInfo contract
test is updated to the corrected metadata.

bitchatTests/ProtocolContractTests.swift

@@ -50,14 +50,19 @@ private final class DefaultTransportProbe: Transport {
 struct ProtocolContractTests {
     @Test
     func commandInfo_exposesAliasesPlaceholdersAndGeoVariants() {
-        #expect(CommandInfo.message.id == "dm")
-        #expect(CommandInfo.message.alias == "/dm")
+        // Aliases must match what CommandProcessor actually accepts —
+        // the suggestion panel is the only command-discovery surface.
+        #expect(CommandInfo.message.id == "msg")
+        #expect(CommandInfo.message.alias == "/msg")
         #expect(CommandInfo.message.placeholder != nil)
         #expect(CommandInfo.clear.placeholder == nil)
         #expect(CommandInfo.favorite.description.isEmpty == false)
-        #expect(CommandInfo.all(isGeoPublic: false, isGeoDM: false).contains(.favorite) == false)
-        #expect(CommandInfo.all(isGeoPublic: true, isGeoDM: false).contains(.favorite))
-        #expect(CommandInfo.all(isGeoPublic: false, isGeoDM: true).contains(.unfavorite))
+        #expect(CommandInfo.all(isGeoPublic: false, isGeoDM: false).contains(.help))
+        // Favorites are rejected by the processor in geohash contexts, so
+        // they are suggested only in mesh.
+        #expect(CommandInfo.all(isGeoPublic: false, isGeoDM: false).contains(.favorite))
+        #expect(CommandInfo.all(isGeoPublic: true, isGeoDM: false).contains(.favorite) == false)
+        #expect(CommandInfo.all(isGeoPublic: false, isGeoDM: true).contains(.unfavorite) == false)
     }
 
     @Test