feat: linked in character limit warning

[love25-codes]

Summary

Added a configurable LinkedIn character limit warning that alerts users when their input exceeds the recommended length, helping them create LinkedIn-friendly posts before formatting.

Related Issue

Closes #441

Changes

• Added a configurable LINKEDIN_CHARACTER_LIMIT constant (default: 3000 characters).
• Added a warning message that appears when the input exceeds the configured character limit.
• Updated the character counter logic to dynamically show and hide the warning based on the current input length.
• Styled the warning banner to make it visually distinct while remaining consistent with the application's theme.
• Kept the warning informational only without restricting formatting or editing functionality.
• Preserved the existing character counter, word counter, and formatting features.

Testing

• Added/updated tests
• Tested locally (describe steps below)

Testing Steps

1. Open the LinkedIn Text Formatter and verify that the warning is hidden on initial page load.
2. Enter text with fewer than 3000 characters and confirm that only the character and word counters update without displaying the warning.
3. Paste or type text exceeding 3000 characters and verify that the "⚠ Too Long for LinkedIn" warning appears. Delete characters until the count drops below the limit and confirm that the warning disappears automatically.

Screenshots:

After:

Contributor Details

• Name: Lovepreet Kaur
• GitHub Username: @love25-codes
• Program (SSoC / GSSoC / Hacktoberfest / Other): SSoC

Checklist before requesting a review

• Code follows the project's conventions
• No secrets or .env values are committed
• I have performed a self-review of my code
• Documentation has been updated if needed
• CI passes
• Linked the related issue above

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