feat: separate SQLite database path from output folder

[anderstorstensson]

The output folder is often on a network drive shared between team members, but SQLite databases are not safe on network filesystems due to unreliable file locking. This introduces a separate, configurable database directory that defaults to a local user-level path (tools::R_user_dir("ClassiPyR", "data")), keeping the database safe while the output folder can remain on a network drive.

• Add get_default_db_dir() returning a persistent local directory
• Rename get_db_path() parameter from output_folder to db_folder
• Add db_folder parameter to save_sample_annotations() and rescan_file_index()
• Add Database Folder setting with browse button and network warning
• Update all get_db_path() calls in server.R to use config$db_folder
• Add migration FAQ for transferring annotations between machines
• Update vignettes with new settings table entries and network notes
• Add _pkgdown.yml entry for get_default_db_dir

[codecov-commenter]

Codecov Report

❌ Patch coverage is 92.90323% with 22 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
R/database.R	92.10%	21 Missing ⚠️
R/utils.R	92.85%	1 Missing ⚠️

@@            Coverage Diff             @@
##             main       #7      +/-   ##
==========================================
+ Coverage   87.59%   89.76%   +2.16%     
==========================================
  Files           4        5       +1     
  Lines         379      674     +295     
==========================================
+ Hits          332      605     +273     
- Misses         47       69      +22     

Files with missing lines	Coverage Δ
R/sample_loading.R	97.64% <100.00%> (+0.24%)	⬆️
R/sample_saving.R	97.89% <100.00%> (+0.24%)	⬆️
R/utils.R	82.79% <92.85%> (+0.43%)	⬆️
R/database.R	92.10% <92.10%> (ø)

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