update new feature filter by core/ccf/thupl

[ZeddYu]

This PR expands rank-based filtering and subscription support across the site.

• Added CORE rank filtering end-to-end: homepage filters, tag highlighting, subscription modal, and generated iCal/RSS
  feeds.
• Added THCPL as a first-class filter alongside CCF and CORE, including UI, highlighting, modal summaries, and feed
  generation.
• Reworked rank filters into compact dropdowns and ensured only one dropdown stays open at a time.
• Updated conference cards to highlight selected CCF / CORE / THCPL labels and show Non-CORE / Non-THCPL explicitly.
• Simplified the Subscribe modal copy and clarified that generated links reflect the current filter combination.
• Added mutual exclusion for Non-CCF / Non-CORE / Non-THCPL versus specific ranks, with local state normalization for
  old selections.
• Extended iCal/RSS filename generation to support combined ccf + core + thcpl + sub filters.

Validation: cargo check and trunk build --release --dist .tmp-trunk-dist passed.

Related issue: #854

[mergify]

Welcome to ccf-deadlines!
We're delighted to have you onboard (p≧w≦q)

[mergify]

Great job!
Thank you @ZeddYu for your contribution!