refactor: use shared exclusion utilities in general_exclusions

[js3110]

Issue

Closes #1126

Description

Refactors general_exclusions.R to use the shared utilities from exclusion-utils.R (introduced in #1125), replacing duplicated inline logic. Net -23 lines, no behavior change.

Definition of Done

• general_exclusions.R uses rehydrate_exclusions(), observe_remove_buttons(), and clean_exclusion_list() instead of inline implementations
• Existing general exclusion functionality (add, remove, persist, restore) works identically

How to test

1. Upload data, go to General Exclusions, add exclusions with reasons
2. Verify remove buttons work (click X to remove an exclusion)
3. Download settings YAML, reload, upload settings — verify general exclusions are restored
4. Verify no regressions in NCA results when exclusions are active

Contributor checklist

• Code passes lintr checks
• Code passes all unit tests
• New logic covered by unit tests
• New logic is documented
• App or package changes are reflected in NEWS
• Package version is incremented
• R script works with the new implementation (if applicable)
• Settings upload works with the new implementation (if applicable)

Notes to reviewer

• Depends on feat: persist PP exclusions in settings YAML download and upload #1125 — merge that first.
• Pure refactor, no functional changes. The shared utilities are already tested in test-parameter_exclusions.R."