feat: add predefined tag selector with Other input and normalize tag …

[aryanpatil-07]

Adds a predefined tag selector to the Task modal (selectable chips + "Other" custom input) and makes category filtering robust by normalizing tag strings (trim + case-insensitive). Also relaxes the backend Task model tag validation so custom tags are accepted.

🔗 Related Issue

Closes #340

Changes Made

• UI: added selectable predefined tag chips in the task modal (e.g., Homework, Routine, Creative) — tap to toggle selection.
• UI: added an "Other" chip that reveals a small input to add a custom tag; custom tags appear as removable chips.
• UX: prevent empty or duplicate tags (duplicate check is case‑insensitive).
• Filtering: tag filtering now normalizes values when matching (trim + toLowerCase) so filters match regardless of casing or surrounding spaces.
• Filtering behavior: multi‑select filters work as OR across selected categories (task shown if any selected category matches any of its tags).
• Backend: relaxed strict tag validation so arbitrary/custom tag strings are accepted and persisted.
• Edge cases handled: ignores blank custom tags, avoids adding duplicates, and shows removable custom tag badges.

[aryandas2911]

@aryanpatil-07 pls do one change, the tags should be same as the categories in the filter by category so that they can be sorted and filtered pls do the change thank you

[aryanpatil-07]

Hey @aryandas2911, made the required changes.
Please let me know if any other changes are required.
Thank you