Feature Request: Support additional placeholder variables in the system prompt

Describe the feature you'd like to request

Summary

Currently, the admin-configurable "Chat User Instructions" (system prompt) for both chat completions and title generation only supports a single placeholder: {user}, which resolves to the current user's display name (see lib/Controller/ChattyLLMController.php, str_replace('{user}', $user->getDisplayName(), $userInstructions)).

It would be useful to support a small set of additional placeholders so admins can write more context-aware and personalized system prompts without needing code changes for every use case.

Motivation

Right now the system prompt is fairly static aside from the display name. Being able to inject a few more pieces of contextual information (date, language, login name, etc.) would let admins:

• Localize/adjust behavior based on the user's preferred language
• Reference the current date/time for time-sensitive tasks (e.g. "today is {date}")
• Distinguish between a user's display name and their login/username

Describe the solution you'd like

Proposed variables

Additional context

• Substitution currently happens via simple str_replace() calls in lib/Controller/ChattyLLMController.php (used in newSession(), generateTitle(), and checkTitleGenerationTask()), so adding more placeholders would follow the same pattern.
• The available placeholders are documented inline in the admin settings UI (src/components/AdminSettings.vue) via NcNoteCard hint text below each prompt field — this documentation should be updated to list any newly added variables.
• Open question: should placeholder resolution be strict (only replace exact known tokens) or should unknown {...} tokens be left untouched/escaped to avoid breaking existing prompts that happen to contain literal curly braces?

Describe alternatives you've considered

None

Assisted-by: ClaudeCode:claude-fable-5

Start in lib/Controller/ChattyLLMController.php by tracing placeholder substitution in newSession(), generateTitle(), and checkTitleGenerationTask(). Review the user and localization APIs needed for the proposed values, then update the prompt hints in src/components/AdminSettings.vue. Done means the supported placeholders work consistently in each flow and the admin documentation lists them, with the unknown-token behavior clarified.