fix(tui): replace provider template chip row with a template field and grouped picker

[SaladDay]

Closes #416. Thanks @LeonardoTan19 for the report and the offer to help — this ended up as a UX redesign rather than a scroll fix, details below.

Problem

The provider Add form rendered every template as one horizontal chip row. With 3 built-ins + 10 sponsor presets (~135 columns), the row overflows any realistic terminal width, and the overflow was silently clipped: the selected chip — and the sponsor entries — could become invisible and unreachable.

Approach

Instead of adding horizontal scrolling (which would keep most sponsors hidden behind the fold), template selection now follows the same pattern the Usage Query page already uses — a field whose Enter opens a picker:

• Template is a regular field at the top of the Fields table: chip-styled current selection + dim ▾ (N) count, followed by a divider. Label-first degradation at narrow widths (count dropped first, then the caret, then the label truncates). Nothing can overflow horizontally anymore.
• Enter opens a grouped vertical picker (ProviderTemplatePicker): Built-in / Sponsors sections (Codex's DeepSeek stays under Built-in), ↑/↓ skip section headers, selection kept in view via the shared scroll window on short terminals.
• Apply semantics unchanged: apply_template with existing provider ids (id de-confliction covered by a test), common-snippet quick-config refresh, warning toast on failure — then the cursor lands on the first real field. Esc changes nothing, including typed API keys and usage-query settings.
• Simpler focus model: the dedicated Templates pane and focus stop are gone; Tab cycles Fields ↔ JSON preview (Codex: Fields → Auth preview → Config preview). The MCP form and the CLI chooser are untouched.
• Copy-provider flow opens on the first real field instead of the template row, so copied values are not one accidental Enter-Enter away from being reset.
• ? help rewritten (EN + ZH) to document the picker keys, the sponsor grouping, and — precisely — what applying a template does and does not reset (it deliberately avoids over-promising: some previously typed values survive a template switch, so the help now says not to rely on templates to clear secrets).

Screenshot-able flow

Providers → a → first row is Template [ Custom ] ▾ (13) → Enter opens the grouped list → pick → cursor lands on Name.

Validation

• cargo fmt --check clean; no new clippy warnings in touched files.
• cli::tui suite: 1799 passed (≈20 new behavior tests: no-apply-on-open, header skipping incl. Codex display-order divergence, id de-confliction, Esc purity with typed secrets, warning-toast path, scroll-into-view at 100×13, narrow-width degradation ladder, copy-flow initial focus, help routing). Full-suite failures are byte-identical to the pre-change baseline on this machine (pre-existing umask/sandbox failures, none in cli::tui).
• Change went through multiple rounds of independent blind review (correctness, regression, security, UX, i18n accuracy, test quality) until convergence; the ? help text was additionally fact-checked claim-by-claim against apply_template.

Note: cargo clippy currently cannot exit clean on main due to a pre-existing reversed_empty_ranges deny error in src/cli/tui/ui/home_chart.rs:806 (untouched here) — worth a separate small fix.