Attachment limits: clearer Plan-tab rows and coherent

Found while visually checking Task 7143 (configurable max file size and task storage via Operator Settings → Plan → Org). 1. On an org's Plan tab, the "Max file size (MB)" row reads "100 · Platform · 0 of 100 in use". Usage of a per-file cap isn't a running total, so show "largest file N MB" instead of "N of 100 in use". 2. The default "Attachment storage per task" (25 MB) is below the default "Max file size" (100 MB). Files between 25 and 100 MB pass the file-size check, then get refused by the task quota with a different message. Raise the per-task default to at least the file-size default, or refuse with one message that names whichever limit is lower. 3. The Plan-tab Limits rows show only "Platform" or "Plan" as the source. When a plan sets the attachment quotas, the row should name the plan, like the feature rows do ("From Business"). 4. The plan list's caps summary (`BillingPlansSection` `capsText`) doesn't mention the two new attachment fields. Show them when a plan sets them. 5. The org Limits dialog says "0 means no limit" for Max file size, but 0 actually means the 5120 MB ceiling. Say "0 means 5120 MB" for that row. Out of scope: changing how the limits are resolved (override → plan → knob) or the 5 GB ceiling. Done means: - Each item above is either visible on the live console (screenshot) or the default is changed with a test. - `./dc check quick` is green.

All 5 Plan-tab attachment-limit items shipped (largest-file row, plan name as source, caps summary, "0 means 5120 MB", task-storage default 25→100 MB with a test). Merged as #5833 (25d86c7) after the lane's full gate; deploy superseded. Follow-up draft #7182.