refactor: remove unnecessary important declarations

[rdlabo]

Summary

• remove normal-cascade important declarations where selector specificity is already sufficient
• use a narrowly scoped collapsing-header selector where additional specificity is required
• reorder range mixins so the intended transform wins without important
• reduce searchbar important usage from three broad declarations to one centered-placeholder state required to override Ionic inline padding

Compatibility

• this PR is intentionally stacked on refactor: prefer CSS custom properties over Shadow Parts #133 so it contains only important cleanup
• the resulting combined source is identical to the previously reviewed and Mac-verified implementation
• no demo-specific selectors or fixture assumptions
• manager and independent acceptance reviews completed with no blocking findings

Validation

• npm run build
• Prettier check for src
• git diff --check
• affected Mac Chrome light/dark screenshots matched the before-change baseline
• verified collapsing-header transition and pressed single/dual range computed styles

Linux Playwright snapshots were not modified.

[github-actions]

Playwright test results

 90 passed

Details

 90 tests across 2 suites
 1 minute, 53 seconds
 130ac69
 This detailed result covers Ionic 9 only. Ionic 8 runs against the same screenshots in a separate matrix job; check the workflow run for both results. To update the screenshots, comment with /update-screenshots.

[github-actions]

📊 Ionic 9 Playwright Test Report

View the detailed Ionic 9 report: https://rdlabo-dev.github.io/ionic-theme-ios26/pr-134/

Ionic 8 runs against the same screenshots in a separate matrix job. View both results in the workflow run.