fix: clean error instead of panic when no config can be found

[usefulish]

Config::default() unwraps when the Syncthing config cannot be found, so the daemon exits with a panic and backtrace instead of a readable error. Propagate the error through parse() instead, preserving the existing context message.

No test: simulating "no config anywhere" requires mutating process-global env vars, which would race with parallel tests. Verified by hand that the full error chain is printed and the process exits 1:

WARN  [stfed::config] Unable to find config file, using default config
Error: Failed to read local config

Caused by:
    0: Unable to guess stfed configuration field values from Synthing config, please write a config file
    1: Unable fo find Synthing config file

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