1761: Distance controls and labels disable for epi-only networks

[glstott]

Summary

Addresses #1761 by making molecular-distance controls respond consistently to whether the loaded data can provide distance information.

• Added shared detection for available distance data, including distance-bearing links, sequences, trees, and matrices.
• Disabled and visually muted irrelevant distance controls in pre-launch File Settings and post-launch Global Settings.
• Added explanatory messaging when controls are unavailable.
• Hid the 2D Network threshold statistic for epi-only datasets.
• Removed Distance Metric and Threshold rows from the Network Statistics UI when distance data is unavailable.
• Updated CSV and XLSX Network Statistics exports to follow the same rule.
• Preserved distance controls for FASTA, Newick, and restored sessions even when no distance links are currently materialized.
• Added synthetic-data Cypress coverage for epi-only, link-distance, FASTA, Newick, live capability changes, session restoration, and Network Statistics exports.
• Updated related threshold and minimum-cluster-size regression expectations.

Scope expansion

The original issue exposed the threshold control when no molecular-distance links were available. The scope was expanded because the same capability affected several connected workflows: pre-launch settings, global filtering, 2D statistics, Network Statistics, and exported reports.

Fixing only the original control would have left contradictory states—for example, disabling the threshold in Global Settings while still showing it in statistics or including it in exported reports. The broader change establishes one capability rule across these surfaces while distinguishing truly epi-only data from molecular sources whose distance links have not yet been generated.

Verification

• Focused Angular unit tests: 9 passed
• Focused Cypress feature spec: 4 passed, 0 failed
• Development build: passed
• Cypress verified that epi-only CSV and XLSX exports omit Distance Metric and Threshold.
• Existing molecular-data export coverage verifies those rows remain present.