Session resource grid: controls and legend do not explain what they encode

Follow-up from the FR-3571 stack review.

Several parts of the grid are not self-explanatory to a first-time user:

- The mode / metric / memory-unit segmented controls carry no label or prefix, so it is unclear what the segments select. Adding a prefix inside the segment row may itself add confusion, so the placement needs a design pass.
- The memory unit values (`1GiB`, `2GiB`, ...) are not intuitive without stating that one square equals that amount.
- The legend does not say what it maps to. It colors the unit squares only, while the group plate border/background is a separate, per-session, user-assignable color — the two are easy to conflate.
- A partially filled square means the session was allocated less than one square is worth (e.g. one square = 4GiB, allocation = 1GiB). Nothing in the UI conveys this.

**Proposal**: add labels/help text to the grid controls and a short explanation of the encoding (square color vs plate color, unit size, partial fill).

Reported by Sujin Kim.

JIRA Issue: FR-3582

Start by locating the session resource grid and inspect its mode, metric, and memory-unit controls, legend, squares, and session plates. Review FR-3571 and the proposal, then make the encoding understandable by distinguishing square color from plate color, stating the unit size, and explaining partial fills; confirm the controls and legend are self-explanatory in the UI.

Written by the indexing model from the issue text.