Add text_command + refresh live-text keys

Introduce a per-key `text_command` + `refresh` feature: the service
periodically runs a shell command and renders its stdout as the key's
text overlay, compositing over the base icon (or a black background for
text-only keys) each tick. `text` acts as the static fallback shown
before the first run or when the command errors/outputs nothing. A
refresh goroutine owns the key's image and also handles an optional poll
block (choosing icon_true/icon_false per tick). Adds `$(...)` sugar in
`text` as shorthand for `text_command`.

- internal/config: add TextCommand/Refresh fields to KeyConfig
- cmd/streamdeck: refreshTextKey goroutine, runTextCommand, unwrapCmdSubst
- config.example.yaml: document the feature with a `flow` pomodoro
  live-countdown example (status --short, toggle/skip/stop/gui)
- modules.example.yaml: example config updates

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01TEoLyNbtbjbDq7aTWAymUG

config.example.yaml

@@ -61,3 +61,48 @@ keys:
   #     command: pactl get-source-mute @DEFAULT_SOURCE@
   #     interval: 2s
   #     match: "yes"
+
+  # --- Dynamic text from command output ---
+  #
+  # text_command: a shell command whose stdout is rendered as the key's label,
+  #   re-run every `refresh` interval (default 1s; floored to 250ms). Newlines
+  #   in the output become label line breaks; trailing whitespace is trimmed.
+  #   `text` (if set) is the static fallback shown before the first run or when
+  #   the command errors / outputs nothing. Composited over `icon` each tick.
+  #
+  # NOTE: use the ABSOLUTE binary path — the service runs with a minimal PATH.
+  #   Apple Silicon Homebrew installs may live at /opt/homebrew/bin/flow instead.
+  #
+  # Pomodoro (`flow`) live countdown — press toggles start/pause:
+  # 8:
+  #   icon: pomodoro.png                                   # base icon; text on top
+  #   text_command: "/usr/local/bin/flow status --short"   # prints e.g. WORK\n24:12 (real newline)
+  #   refresh: 1s
+  #   text_color: "#FFFFFF"
+  #   command: "/usr/local/bin/flow toggle"
+  #
+  # Separate icon-swap indicator (running vs paused):
+  # 9:
+  #   icon_true: running.png
+  #   icon_false: paused.png
+  #   command: "/usr/local/bin/flow toggle"
+  #   poll:
+  #     command: "/usr/local/bin/flow status"             # prints: state=running phase=work remaining=24:12
+  #     interval: 1s
+  #     match: "state=running"
+  #
+  # Control keys:
+  # 10:
+  #   icon: skip.png
+  #   command: "/usr/local/bin/flow skip"
+  # 11:
+  #   icon: stop.png
+  #   command: "/usr/local/bin/flow stop"
+  # 12:
+  #   icon: gui.png
+  #   command: "/usr/local/bin/flow gui"
+  #
+  # A text-only live clock (no icon → white text on black), refreshed each second:
+  # 13:
+  #   text_command: "date +%H:%M:%S"
+  #   refresh: 1s