feat(tui): add and remove providers from the Providers tab (PR-4)

[xjoker]

Fourth slice of custom-provider support (方案 A). Makes the Providers tab interactive.

Stacked on PR-3 (#69) — base branch is cursor/provider-tui-panel-5a91; retarget down the stack as they merge.

What's in this PR

On the Providers tab:

• a — add wizard: a multi-step, single-line prompt walking through alias → base URL → model → API key, reusing the existing input pattern. The API-key step is masked in the status bar and the key is never echoed. Each step validates (alias rules + collision with existing providers/accounts, http(s):// base URL, non-empty model/key). On completion it derives env_key (CODEX_SWITCH_<ALIAS>_KEY) and provider_id, saves via provider::save, reloads, and selects the new row.
• d — remove: deletes the selected provider after a y/n confirmation (ConfirmAction::RemoveProvider).
• Keys route per tab; the wizard consumes raw, case-sensitive input (aliases/URLs/keys are case-sensitive). A tab-aware footer shows the Providers key hints (j/k nav · a add · d remove · Tab accounts · h help · q quit).

Now both entry points requested — CLI (PR-1) and TUI — can add providers.

Tests

• provider_add_wizard_collects_fields_and_saves — drives the wizard key-by-key (hermetic CODEX_SWITCH_HOME/CODEX_HOME) and asserts the saved profile (base_url/model/derived env_key/key/wire_api), tab switch, and reload.
• provider_add_wizard_stays_on_step_for_a_bad_base_url — invalid base URL keeps the wizard on that step.
• request_and_confirm_remove_provider_deletes_it — d asks for confirmation; y removes and reloads.
• cargo fmt --check, cargo clippy --all-targets --all-features -- -D warnings, cargo test --all — all pass, 0 failed.

Manual demo (real TUI, verified via video review)

Empty Providers tab → a → typed alias/base URL/model/masked API key → provider appears in the table (no secret) → d → Remove provider 'openrouter'? (y/n) → y → table empty again.

tui_provider_add_remove_wizard_demo.mp4

API-key step masked:

add wizard API key masked

Provider added (no secret shown):

provider added

Remove confirmation:

remove confirmation

Note: CI Format and audit will be red until #63 (webbrowser RUSTSEC bump) merges to dev; pre-existing and unrelated.

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