feat: Add a data conflict resolution modal

[mhlas7]

When AeroDataBox returns data that conflicts with the current values, open a merge conflict modal and ask the user which values they want to keep. If there are no conflicts, this modal will not display and values will be filled in maintaining the existing behavior.

Note

Add per-field conflict resolution modal for flight lookup results

• Adds FlightMergeConflictModal.svelte, a modal that shows conflicting fields side-by-side (current vs. fetched) with bulk and per-field selection, defaulting to fetched values.
• Adds merge-fields.ts with domain types and functions (buildMergeFieldStates, getFetchedSources, isConflict) to classify fields as conflicting, equal, or fetched-only.
• Rewrites applyLookupResult in FlightNumber.svelte to auto-apply non-conflicting fetched values and show the modal only when conflicts exist.
• Behavioral Change: the previous blanket confirm-and-overwrite prompt is removed; conflicting fields are now preserved unless the user explicitly selects the fetched value and clicks Apply.

^{Macroscope summarized 1d4fec6.}

[github-actions]

Label error. Requires exactly 1 of: changelog:.*. Found: Web

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

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