fix(tts): strip Markdown syntax before sending text to TTS engines (#5560)

* fix(tts): strip Markdown syntax before sending text to TTS engines

Chat responses are rendered as Markdown but the TTS components piped the
raw response into Piper / the browser's `SpeechSynthesis` API. The
synthesizer reads every special character literally — `**bold**` becomes
"asterisk asterisk bold asterisk asterisk", `# Heading` becomes "pound
heading", code fences are read backtick-by-backtick, and bullet lists
become "hyphen item". The result is unintelligible whenever the assistant
includes any formatting, which is most of the time.

This commit adds a small `messageToSpeech` helper that converts a
Markdown chat message into plain text suitable for TTS:

- fenced code blocks and images are dropped (nothing useful to read)
- inline code and link labels keep their text content
- emphasis markers, headings, blockquote markers, list markers, and
  horizontal rules are stripped while preserving the underlying words
- HTML tags are removed but their text content kept
- table pipes become commas so rows read naturally

The helper is regex-based — no new dependency — and is wired into both
the native (`SpeechSynthesis`) and Piper TTS components in
`WorkspaceChat/ChatContainer/ChatHistory/HistoricalMessage/Actions/TTSButton`.

Closes #5557.
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Co-authored-by: Timothy Carambat <rambat1010@gmail.com>