Promote strictness in ui-tui-opentui-v2:
- eslint: @typescript-eslint/no-non-null-assertion: error (with a test
override block keeping `!` in *.test.ts/tsx fixtures), and promote
unused-imports/no-unused-vars warn → error.
- tsconfig: add noUnusedLocals + noImplicitReturns.
Remove all 24 production `!` non-null assertions by replacing each with
a real guard / default / early-return, preserving rendered behavior:
- gateway/client.ts: read the pending entry once and guard (vs has()+get()!).
- logic/theme.ts: guard the parseHex regex match; `?? 0` on the always-
in-bounds XTERM_6_LEVELS lookups; restructure backgroundLuminance to
branch into a typed tuple and use charAt() for the 3-digit hex expand.
- view/homeHint.tsx: use Solid's <Show>{value => …} callback form to
narrow info().model / info().cwd instead of `!`.
- view/reasoningPart.tsx: guard the regex match before slicing m[0].
- view/statusBar.tsx: read model/cwd/pct into locals + guard; `?? 0` on
the showBar()-guarded context-bar percentage.
- view/toolPart.tsx: guard the single-arg entry; `?? 0` on the
duration-guarded fmtDuration.
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