FE: Change CEL filter managing logic

[Vixtir]

• Breaking change? (if so, please describe the impact and migration path for existing application instances)

What changes did you make? (Give an overview)

1. The Save Filter button has been removed.
2. CEL filters are now saved by default. Clicking the Add Filter button saves the filter, applies it, and closes the sidebar.
3. The display name of a CEL filter is now optional. If omitted, it will be automatically generated using the first 32 characters.
4. Applied filters can be restored from localStorage. Filter uniqueness is determined by the combination of cluster name + topic name.
5. Clicking the ✕ (cross) on an applied filter in Topic → Messages now only unapplies the filter; it does not delete it from the saved filters.

Is there anything you'd like reviewers to focus on?

How Has This Been Tested? (put an "x" (case-sensitive!) next to an item)

• No need to
• Manually (please, describe, if necessary)
• Unit checks
• Integration checks
• Covered by existing automation

Checklist (put an "x" (case-sensitive!) next to all the items, otherwise the build will fail)

• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation (e.g. ENVIRONMENT VARIABLES)
• My changes generate no new warnings (e.g. Sonar is happy)
• 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
• Any dependent changes have been merged

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