reco-gui: default calibration preference + overwrite confirmation

[RufanMelfor]

Summary

• Adds a Default Calibration file path to reco-gui's Preferences dialog, used as a fallback whenever no calibration is otherwise loaded/picked for a session.
• The fallback is applied in try_init_and_update - the single point every left/right/calibration pick path already converges on before initialization - so it covers "no calibration available" generically instead of needing per-pick-site handling. An explicit pick always takes priority over the default.
• Saving over the file currently configured as the default now shows a confirmation modal ("Overwrite default calibration?") instead of silently overwriting it, since the whole point of a default is to stay a stable fallback rather than get mutated by whatever the current session happens to be editing.

Why

A default calibration is useful for rigs/setups that are reused across many sessions (e.g. a fixed camera rig at a home venue) - without it, every new session with no calibration picked requires re-selecting the same file. But since calibration_path can point at that same file during an editing session, an unprotected "Save Calibration" could silently clobber the shared default with in-progress, session-specific tweaks - the confirmation step exists specifically to prevent that.

Implementation

• GuiSettings::default_calibration_path: Option<PathBuf> (crates/reco-gui/src/settings.rs), following the exact shape of the existing ai_model_path/recording_folder fields, plus a default_calibration() accessor that only returns the path if it still exists on disk (mirrors how MRU entries are already validated elsewhere in this file).
• Preferences dialog: new "Default calibration" row (LineEdit + "Browse…") under AI model, wired via prefs-default-calibration-path + pick-prefs-default-calibration, prefilled/persisted in on_open_prefs_dialog/on_save_prefs.
• AppState::is_default_calibration() compares the currently-loaded calibration_path against the configured default. on_save_calibration checks this first; if true, it opens a new overwrite-default-cal-warning-open modal instead of writing immediately. Confirming calls a separate confirm-save-calibration callback that performs the actual write (shared do_save_calibration helper avoids duplicating the save/toast logic between the two paths).

Test plan

• cargo build -p reco-gui - clean
• cargo fmt --all -- --check - clean
• cargo test -p reco-gui settings:: - 5/5 pass (3 new tests covering default_calibration(): unset, stale/deleted path, existing path)
• cargo clippy -p reco-gui --all-targets -- -D warnings - no new warnings; the 4 errors present are pre-existing on main (reco-core dead-code/unsafe-ptr-arg issues, same ones tracked by Clear pre-existing clippy -D warnings failures on Windows #423) and unrelated to this change
• Manual GUI verification (set a default in Preferences, start a session with no calibration picked, confirm it loads; edit and Save, confirm the warning modal appears and Cancel/Overwrite both behave correctly) - confirmed working by @RufanMelfor

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[RufanMelfor]

I have read the CLA Document and I hereby sign the CLA