Fix timer button negative countdown bug and enhance example app

[ProjectAJ14]

Summary

• Fix negative countdown bug: Clamp timeOutInSeconds to 0 when negative, preventing display of values like "-5s"
• Fix double-decrement on reset: Cancel pending timer before resetting counter in _onPressed, preventing a spurious extra tick
• Add guard in _updateTime: Early return when counter is already <= 0
• Code cleanup: Extract _safeTimeout getter to deduplicate clamping, remove unnecessary async on timer callback
• Add 2 new tests: Verify negative timeOutInSeconds input is clamped and counter never goes below 0
• Enhance example app: Restructure into app.dart, home_page.dart, theme.dart with Material 3 theming

Closes #24

Test plan

• All 33 existing + new tests pass (flutter test)
• flutter analyze passes with no issues
• Manual: verify timer button shows "0s" (not "-5s") with negative timeOutInSeconds
• Manual: verify button enables immediately when timeOutInSeconds is negative
• Manual: verify example app renders with Material 3 theme in both light/dark modes

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

Flag	Coverage Δ
dzod	?
html_rich_text	?
morse_tap	?
ns_intl_phone_input	?
ns_utils	?
timer_button	100.00% <100.00%> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
packages/timer_button/lib/timer_button.dart	100.00% <100.00%> (ø)

... and 63 files with indirect coverage changes

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