TUI: provider create/update form cannot set config keys (e.g., ANTHROPIC_BASE_URL) #1798

Summary

The TUI's "Create Provider" and "Update Provider" forms accept
credentials but provide no UI or code path for the provider's
config map. As a result, providers authored via openshell term
always carry an empty config, which makes it impossible to use the
TUI to author a working provider that points at any non-default
upstream URL (e.g., ANTHROPIC_BASE_URL, OPENAI_BASE_URL,
custom-routed proxy endpoints, in-cluster bridges, etc.).

The CLI (openshell provider create --config KEY=VALUE) handles
this correctly — only the TUI surface is affected.

Reproduction

1. openshell term
2. Providers screen → Create Provider
3. Type: anthropic, fill in ANTHROPIC_API_KEY=sk-ant-…
4. Save.
5. openshell provider get <name> — observe config is empty
   (no ANTHROPIC_BASE_URL).

Same applies to the Update Provider form for an already-created
provider: editing it via TUI clobbers the config (since the form
sends an empty map; see source below).

Source location (upstream main at time of writing)

crates/openshell-tui/src/lib.rs:

• spawn_create_provider (~L1581) — line ~L1619:
  config: HashMap::default(),
• spawn_update_provider (similar) — line ~L1711:
  config: HashMap::default(),

In both spots the proto request is built with an unconditionally-empty
config map; the form structs themselves don't carry config-key
state at all.

Expected

The Create/Update Provider forms should expose the same config map
the CLI's --config flag exposes — likely as a section parallel to
the existing credentials section in the form, with the same
add/edit/remove key/value affordances.

For the Update form specifically: until the field is editable, the
form should at least preserve the existing config from the
fetched provider rather than overwriting with empty (current behavior
is destructive — TUI-edit of an existing provider wipes its
URL/config).

Why it matters

• Any setup that points an anthropic/openai/etc. provider at a
  non-default URL (in-cluster proxy, regional endpoint, custom
  router, BYO inference gateway) cannot be configured or maintained
  through the TUI.
• The Update Provider clobber is silently destructive — a TUI user
  who edits credentials on an existing provider unknowingly removes
  its ANTHROPIC_BASE_URL.

Suggested fix sketch

1. Extend CreateProviderForm / UpdateProviderForm with a config: Vec<(String, String)> (or IndexMap<String, String>) and reuse
   the same UI patterns the credentials section uses for keyboard
   add/edit/remove.
2. Plumb the populated map into the two spawn_* functions in place
   of HashMap::default().
3. For the Update form, hydrate the new field from the fetched
   provider on form open, so the in-memory edit is non-destructive
   even if the user doesn't touch it.

Happy to take a stab at this if it's a useful contribution.