ponymail-mcp: add limit/offset pagination to search_list (#12)

The search_list tool previously rendered the first 30 email summaries
unconditionally, then printed "... and N more emails" with no way to
reach those N entries without re-querying the backend (and getting the
same first 30 back). For LLM clients doing list-wide analysis, the
remainder was effectively invisible.

This change adds two optional parameters:

  - limit  (int, 1..200, default 30) — caps the rendered window
  - offset (int, >= 0,  default 0)   — skips the first N entries

The backend call is unchanged; both parameters only affect which slice
of the already-fetched result set is formatted into the response. The
default behaviour (no params) is identical to before.

The trailing summary is upgraded from a static "... and N more" to an
explicit "Showing X-Y of Z" plus the exact next-page offset when more
results remain, so the caller can page deterministically.

Signed-off-by: André Ahlert <andre@aex.partners>