Minor cleanups in App: input jank, accept extensions, redundant boolean

[rmuratov]

Closes #33

Three small items from the code review:

• Large-paste jank: the source map textarea state is now only updated after parsing. A pasted multi-megabyte map no longer lands in the controlled value for one frame; React's controlled-value restoration keeps the textarea visually empty while the map parses, and on failure the text reappears together with the warning so it can still be edited. (An explicit event.target.value = '' reset for the rejected-duplicate case turned out to be unnecessary for the same reason — verified by a dedicated test.)
• accept: the file picker now also accepts .json, which source maps are often stored as.
• setIsSourceMapInputError(Boolean(text) && true) → setIsSourceMapInputError(Boolean(text)).

Tests

Three new tests: the pasted text must not be rendered into the textarea while parsing (an unrelated re-render mid-parse syncs the controlled value, which must be empty), a rejected duplicate leaves the textarea empty, and the file picker advertises .map,.txt,.json. The first and third fail on main without the fix (verified via git stash).

Coverage stays at 100%.

[cloudflare-workers-and-pages]

Deploying sourcemap-tools with    Cloudflare Pages

Latest commit:	ad372e0
Status:	✅  Deploy successful!
Preview URL:	https://40793430.sourcemap-tools.pages.dev
Branch Preview URL:	https://app-minor-cleanups.sourcemap-tools.pages.dev

View logs

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (1f5efa9) to head (ad372e0).

Additional details and impacted files

@@            Coverage Diff            @@
##              main       #41   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            9         9           
  Lines          210       210           
  Branches        55        54    -1     
=========================================
  Hits           210       210           

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.