feat(wallet): /pay slash command parser and composer integration (Task 5)

Implements Task 5 of Phase 1:

New files:
- ParsedPayCommand.kt: Sealed interface for parse results
  - WithAddressRecipient: Pay to Cardano address
  - WithMatrixRecipient: Pay to Matrix user (requires lookup)
  - AmountOnly: Amount specified, prompt for recipient
  - Empty: Open payment flow with no prefilled data
  - ParseError: Parse error with human-readable reason

- SlashCommandParser.kt: Full /pay command parser
  - Handles: /pay, /pay 10, /pay 10 ADA, /pay 10 tADA
  - Matrix recipients: /pay 10 ADA @user:server
  - Cardano addresses: /pay 10 ADA addr1...
  - Validates amounts (decimal support, max supply check)
  - Validates addresses (prefix, length, network match)
  - Comprehensive error messages

- SlashCommandParserTest.kt: 40+ unit tests covering all patterns

Modified files:
- ResolvedSuggestion.kt: Added Command type for slash commands
- SuggestionsProcessor.kt: /pay shows as autocomplete suggestion
- MarkdownTextEditorState.kt: Command insertion in text editor
- MessageComposerPresenter.kt: Command handling in InsertSuggestion

Note: MessageComposerPresenter sendMessage interception deferred to
Task 6 (requires PaymentFlowPresenter for navigation).

libraries/textcomposer/impl/src/main/kotlin/io/element/android/libraries/textcomposer/mentions/ResolvedSuggestion.kt

@@ -32,4 +32,12 @@ sealed interface ResolvedSuggestion {
             size = size,
         )
     }
+
+    /**
+     * A slash command suggestion (e.g., /pay).
+     */
+    data class Command(
+        val command: String,
+        val description: String,
+    ) : ResolvedSuggestion
 }

libraries/textcomposer/impl/src/main/kotlin/io/element/android/libraries/textcomposer/model/MarkdownTextEditorState.kt

@@ -77,6 +77,15 @@ class MarkdownTextEditorState(
                 this.text.update(currentText, true)
                 this.selection = IntRange(end + 1, end + 1)
             }
+            is ResolvedSuggestion.Command -> {
+                // Insert the command text with a trailing space
+                val commandWithSpace = "${resolvedSuggestion.command} "
+                val currentText = SpannableStringBuilder(text.value())
+                currentText.replace(suggestion.start, suggestion.end, commandWithSpace)
+                val newCursorPosition = suggestion.start + commandWithSpace.length
+                this.text.update(currentText, true)
+                this.selection = IntRange(newCursorPosition, newCursorPosition)
+            }
         }
     }