fix: avoid panic when filesystem watcher hits permission errors

[leno23]

Summary

• Replace expect calls in the notify watcher setup with error handling.
• When watch setup fails (e.g. permission denied on a subdirectory owned by another user), log a warning and continue without file notifications instead of panicking.
• Log debouncer errors in the forwarder loop instead of ignoring the Err variant.

Test plan

• cargo build
• Manual: git init, create root-owned dir per GitUI was close due to an unexpected panic (file: "src/watcher.rs") #2900, run gitui --watcher — should start without panic (auto-refresh may be disabled)

Fixes #2900

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