fix(stt): filter None values in config.save() to fix silent TOML serialization failure

[Kyotani-Hub]

Summary

Config.save() silently fails (returns False) for every user with default settings because the input_device field defaults to None, and TOML has no null type.

When tomli_w.dump() encounters None in the data dict, it raises a TypeError. This exception is swallowed by the bare except Exception: return False block, so the failure is completely silent — no config file is ever written after a fresh install.

Root Cause

In stt/src/elevenlabs_stt/config.py, the save() method constructs the TOML data dict with all fields unconditionally:

data = {
    "elevenlabs-stt": {
        ...
        "input_device": self.input_device,  # None by default
        ...
    }
}

tomli_w faithfully follows the TOML spec, which has no null/None type, so dump() raises TypeError.

Fix

Filter out None values before passing the dict to tomli_w.dump():

fields = {
    "api_key": self.api_key,
    ...
    "input_device": self.input_device,
    ...
}
data = {"elevenlabs-stt": {k: v for k, v in fields.items() if v is not None}}

When input_device is explicitly set to a device name (a string), it is included normally. When it is None (the default), it is simply omitted from the TOML file — and load() already handles missing keys correctly by using the Pydantic field defaults.

Test plan

• Fresh install: Config().save() returns True and writes a valid config.toml
• With input_device set to a string: field appears in saved TOML
• With input_device as None (default): field is omitted, load() still returns input_device=None
• Round-trip: save() then load() preserves all non-None settings

Fixes #3