fix(tui): make the empty cloud-providers hint bold instead of muted grey

[plombeer31]

What

No cloud providers configured. Press n to add one. was rendered in the same muted grey
as every other empty section, so the one line telling a new user what to do read like
"nothing to see here". It is now bold, in the terminal's default foreground.

Raw bytes from the rendered frame, before and after:

- �[37m  No cloud providers configured. Press n to add one.�[39m
+ �[1m  No cloud providers configured. Press n to add one.�[22m

On "white"

There is no white/foreground token in TuiColors (src/tui/theme/theme.ts:36-52), and the
theme module exists specifically to keep bare colour literals out of components. A hardcoded
color="white" would also be near-invisible on the four light palettes (GITHUB_LIGHT,
CATPPUCCIN_LATTE, GRUVBOX_LIGHT, SOLARIZED_LIGHT).

Leaving color unset renders the terminal's default foreground — white on every dark theme,
readable on the light ones — which is already the pattern in this file (Row deliberately
leaves baseColor undefined) and at 14 other <Text bold> sites in the TUI.

Scope

RowsSection is shared by five sections, so the emphasis is opt-in through a new
emphasiseEmpty prop and only Cloud providers passes it. Local text models, local embeddings,
cloud embeddings and external llama.cpp keep the quiet grey empty state — those are merely
empty, not a call to action.

Testing

• npx vitest run src/tui/components/llm-mode-rows-cloud.test.tsx — 7 passed, including two
  new cases for the zero-provider state, which nothing exercised before
• npm run lint (tsc --noEmit) clean
• llm-panel.test.tsx, manage-panel-fit.test.tsx, llm-fallback-rows.test.tsx — 19 passed,
  so the fixed-height frame assertions still hold

The styling itself is not asserted in the committed test: ink-testing-library renders at
chalk level 0, so SGR sequences never reach lastFrame() — which is why every test in that
file strips ANSI anyway. The bytes quoted above came from raising chalk.level in a throwaway
harness that is not part of this change.