LF-5388(1): Address remaining certification TODOs

[SayakaOno]

Description

• Display loading view when fetching certifications
• Handle isLoading while adding/editing/deleting certifications
• Add success snackbars for adding/editing/deleting certifications
• Preserve original file name
• Fix document preview for private bucket storage

Jira link: https://lite-farm.atlassian.net/browse/LF-5388

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

How Has This Been Tested?

• Passes test case
• UI components visually reviewed on desktop view
• UI components visually reviewed on mobile view
• Other (please explain)

Checklist:

• I have commented my code, particularly in hard-to-understand areas
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• The precommit and linting ran successfully
• I have added or updated language tags for text that's part of the UI
• I have ordered translation keys alphabetically (optional: run pnpm i18n to help with this)
• I have added the GNU General Public License to all new files

[SayakaOno]

Merged without review since we only have one dev this week.