feat: add context meter and file-based system prompts

Two additions to the chat window.

A context meter in the top bar shows how much of the active model's window
the next request will occupy: system prompt, prior turns, attachments and
the current draft. The router does not report a usable context size in
router mode (/props returns n_ctx 0), so the limit comes from ctx-size in
presets.ini. Streaming requests now ask for usage statistics, which makes
the figure exact after each reply at no extra round trip; before that it is
an estimate marked with a leading ~.

The estimate reads low on models with a reasoning budget, since thinking
tokens cannot be known before the reply arrives. The tooltip says so.

System prompts are markdown files in the prompts/ directory beside the
config, one per file, so they can be edited in an editor and kept in
version control. default.md is global, any other file is a named preset
that replaces it rather than adding to it, and a conversation may instead
carry one-off text or opt out entirely. The choice is stored per session so
reopening a chat restores the prompt it was built with.

Prompt names are reduced to safe filenames, so a name like ../../etc/passwd
cannot write outside the prompts directory, and a name that reduces to
nothing is rejected rather than creating a dotfile.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

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