fix: remove hardcoded desktop gateway URL default and allow clearing optional fields

[realchrisolin]

Summary

Fixes two bugs in the connection add/edit dialog that made the Desktop
Gateway URL field effectively unmanageable. This matters because, per #84,
stock Hermes always falls back to legacy REST — reaching a clean REST setup
is the only practical path, and these bugs block it.

1. Hardcoded default

_AddDialog defaulted "Desktop Gateway URL" to a hardcoded developer IP
(http://192.168.1.193/desktop) instead of empty, so a fresh connection
pre-filled a bogus URL.

2. Un-clearable fields

_validateAndSave converted empty optional fields (gatewayPrefix,
dashboardPrefix, desktopGatewayUrl) to null before saving, but
updateConnection's clear* flags (in connection_manager.dart) require a
non-null empty string to detect a clear. With null passed, copyWith's
null ?? oldValue kept the previous value, so these fields silently reverted
to their last value on every edit.

Changes

lib/main.dart only — pass the raw (possibly empty) value through instead of
null-coalescing it, and default the desktop gateway field text to empty.

Verification

Built and installed locally; confirmed the "Desktop Gateway URL" field now
defaults to empty and can be cleared on an existing connection, allowing the
app to fall back to REST chat against a stock Hermes gateway.

Related

• Remote Gateway transport never activates against stock Hermes (turn_recovery capability doesn't exist upstream) + legacy fallback lacks resume re-sync #84 — Remote Gateway transport never activates against stock Hermes; this
  fix makes the documented REST fallback reachable without a working Desktop
  Gateway.

[rusty4444]

Code Review: PR #86 — hermes-android

Verdict: ✅ Approve (no blocking issues)

Reviewed by Hermes Agent (daily automation).

Summary

Two fixes in lib/main.dart _AddDialogState:

1. Removes the hardcoded dev IP default http://192.168.1.193/desktop for the Desktop Gateway URL field — correct, this was a leak of a developer machine address.
2. Passes raw (possibly empty) gatewayPrefix / dashboardPrefix / desktopGatewayUrl values through instead of converting empty→null.

Verification

I verified the second change against the updateConnection contract in lib/core/services/connection_manager.dart (lines ~265–287):

• clearGatewayPrefix: gateway != null && gateway.isEmpty
• clearDashboardPrefix: dashboard != null && dashboard.isEmpty
• clearDesktopGatewayUrl: desktopGateway != null && desktopGateway.isEmpty

Empty→null conversion made clear* flags impossible to trigger on existing connections, so these fields could never be cleared. Passing the raw value through is the correct fix.

Notes

• ⚠️ No CI checks reported on this PR branch (fork PR, cross-repository). Not eligible for auto-merge until CI runs and passes.
• dashboardUsername / dashboardPassword still convert empty→null at call sites; updateConnection derives clearDashboardUsername/clearDashboardPassword from non-null empty values — the same latent bug may exist there. Non-blocking suggestion for a follow-up.