fix: slash commands with arguments bypass command execution (#392)

* fix: intercept slash commands in handleSubmit to pass arguments correctly

When a user types a slash command with arguments (e.g. `/feature implement dark mode`)
and presses Enter, the command was not being intercepted as a slash command. Instead,
the raw text was sent as a regular message to the Claude API, which responded with
"Unknown skill: feature".

Root cause: the command autocomplete menu (useSlashCommands) detects commands via the
regex `/(^|\s)\/(\S*)$/` which only matches when the cursor is right after the command
name with no spaces. As soon as the user types a space to add arguments, the pattern
stops matching, the menu closes, and pressing Enter falls through to handleSubmit which
sends the text as a plain message — completely bypassing command execution.

This fix adds a slash command interceptor at the top of handleSubmit:
- Checks if the trimmed input starts with `/`
- Extracts the command name (text before the first space)
- Looks up the command in the loaded slashCommands list
- If found, delegates to executeCommand() which properly extracts arguments
  via regex and sends them to the backend /api/commands/execute endpoint
- The backend then replaces $ARGUMENTS, $1, $2 etc. in the command template

Changes:
- Added `slashCommands` to the destructured return of useSlashCommands hook
- Added slash command interception logic in handleSubmit before message dispatch
- Added `executeCommand` and `slashCommands` to handleSubmit dependency array

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

* fix: address review — pass rawInput param and cleanup UI state

- Pass trimmedInput to executeCommand to avoid stale closure reads
- Add UI cleanup (images, command menu, textarea) before early return
- Update executeCommand signature to accept optional rawInput parameter

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

---------

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

src/components/chat/hooks/useSlashCommands.ts

@@ -22,7 +22,7 @@ interface UseSlashCommandsOptions {
   input: string;
   setInput: Dispatch<SetStateAction<string>>;
   textareaRef: RefObject<HTMLTextAreaElement>;
-  onExecuteCommand: (command: SlashCommand) => void | Promise<void>;
+  onExecuteCommand: (command: SlashCommand, rawInput?: string) => void | Promise<void>;
 }
 
 const getCommandHistoryKey = (projectName: string) => `command_history_${projectName}`;