feat(viewer): v5 grid 2x2 + datebar calendar + mission label

- NAV viewer v5: grid 2x2 (map + charts), trail slider, play, layer toggles
- Datebar: date picker + datalist, fetches /api/data-dates from :8766
- Mission label shows #NN-folder (X sessions) in green or grey
- Tools: parse_usv_nav, extract_mcap_signals, extract_usv_pwm, merge_nav_usbl, usbl_to_json, check_sync, parse_kogger_usbl
- Vendor: Kogger-Protocol docs

.gitignore

@@ -2,3 +2,8 @@ data/
 output/
 __pycache__/
 *.pyc
+.venv/
+data/
+viewer/data/
+viewer/screenshots/
+screenshots/