fix(import): consistent add-source forms with masked passwords and URL errors

- Mask the Xtream password in add and edit (and the Stalker one in edit)
  behind a shared PasswordVisibilityToggleDirective: one translated
  "Show password" label, state in aria-pressed, type="button".
- Give the Xtream server URL its own mat-error and a neutral hint instead
  of the EPG file error; give the M3U URL a mat-error.
- Use "Playlist title" in every add form, "MAC address" casing, a single
  ellipsis in "Validating portal…" and one "Add playlist" submit label;
  translate the method radiogroup's aria-label.
- Show Stalker refusals inline under the portal URL (role="status", like
  the Xtream connection test), translated in the template and cleared by
  edits; translate the snackbars for outcomes that close the dialog.
- Translate new strings into all locales; reuse the identical Stalker URL
  error translations; fix MAC casing and ellipses; drop unused keys.
- Unit specs per form, edit-dialog spec, new add-source-forms web E2E;
  update E2E locators; UI guidelines Forms section; Stalker contract.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

.changes/import-consistent-add-source-forms.md

@@ -0,0 +1,6 @@
+---
+type: fix
+area: import
+---
+
+The add-source forms now look and behave alike. Xtream and Stalker passwords are hidden, with an eye button to show them, also when you edit a source. Server URL fields explain what is wrong with an address. Every form ends with "Add playlist". When a Stalker portal refuses a source, the reason appears in the form, in your language.