refactor(web): consolidate default/fallback values into named constants

[coderabbitai]

Summary

Several places in the web workspace use magic numbers or inline fallback values instead of named constants. This makes it easy for defaults to diverge across components.

Examples identified

• numberOfJurors defaults to 0 in Jurors.tsx but to 3 in other places.
• disputeKitId falls back to numeric literal 1 / DisputeKits.Classic in multiple locations (SubmitDisputeButton.tsx, SubmitBatchDisputesButton.tsx, Landing/index.tsx, etc.).

Goal

• Introduce a single source-of-truth constants file (or extend an existing one) in the web workspace to hold all default/fallback values (e.g. DEFAULT_NB_JURORS = 3, DEFAULT_DISPUTE_KIT_ID = DisputeKits.Classic, etc.).
• Replace all inline literals/fallbacks across the workspace with references to these constants.

References

• PR: Refactor/dispute kit registry #2267
• Comment: Refactor/dispute kit registry #2267 (comment)

/cc @tractorss