🐛 fix: fix: kernel remove button, CachyOS support, audit fixes, full i18n update

Kernel remove button:
- Always show Remove button for installed kernels (not only when 2+)
- When it's the last backup kernel, keep "Last backup" label + extra
  warning in confirmation dialog

CachyOS kernel support:
- Recognize linux-cachyos and linux-cachyos-lts as valid kernels
- Show in "Installed kernels" with badges (CachyOS, LTS) and
  performance icon
- Exclude from "Available for installation" list
- 4 new tests for CachyOS classification and flags

Audit fixes:
- base_page.py: null check on _get_window() to prevent crash when
  dialog.present receives None
- kernel_manager.py: validate kernel headers availability before
  installation; use specific exception types (OSError, ValueError,
  ParseError) instead of bare Exception
- package_manager.py: thread-safe package cache with threading.Lock
- progress_dialog.py: terminal buffer limit (2000 lines) to prevent
  memory leak on long operations

i18n / translations:
- Regenerate .pot with xgettext (423 → 432 entries, 9 new strings)
- Fix 9 placeholder name translations (cs/et/fi/hu/it/nl/no/pl/ro)
  where {detected}/{installed} were translated to local names
- Fix 22 semantically wrong translations across 12 languages (bg/el/
  et/he/nl/pl/pt/ro/sk/tr/uk/zh) — cleared and retranslated correctly
- Complete fr.po: translate 118 missing strings + add 9 new entries
- Remove 72 obsolete entries from pt-BR.po
- All 29 languages now at 100% coverage (432/432), zero placeholder
  errors, msgfmt syntax validated

Tests: 148 passed (144 existing + 4 new CachyOS tests)

Author: talesam