Show the 180-day session window on the AI Governance pages

What changes for users

Value: 180 days, a constant that is easy to raise later if needed. Shared with Nate in #634979.

• The Recent agent sessions card footnote says it shows sessions from the last 180 days.
• On the Audit events tab, a note appears when a date filter is wider than 180 days, or only "Created before" is set, saying which 180 days are listed.
• Without these notes, the backend change in Bound AI governance session listings to a 180-d... (!265842 - merged) and Bound Duo session artifact listings to a 180-da... (!265854 - merged) would hide older sessions with nothing saying so.

Tells users about the 180-day window that the sessions API now applies, on the "Recent agent sessions" card and on the Audit events tab.

Why

Bound AI governance session listings to a 180-d... (!265842 - merged) limits aiGovernanceSessions to 180 days of sessions, and Bound Duo session artifact listings to a 180-da... (!265854 - merged) does the same for duoWorkflowSessionArtifacts. An earlier start date is moved up to that bound, not rejected. Two surfaces send no window, so older sessions would drop out with no explanation:

• The "Recent agent sessions" card. Its footnote says "Shows the most recent sessions, not filtered by date range".
• The Audit events tab. Its "Created after" and "Created before" tokens accept any dates.

The change

• audit_trail_card.vue: the footnote now reads "Shows the last 180 days, not filtered by date range." It fits on one line in the three-column card. The second part still refers to the dashboard's own 7-day and 30-day control.
• agent_artifacts_table.vue: an info alert appears above the table when "Created after" is more than 180 days before "Created before" (or before today when "Created before" is not set), or when only "Created before" is set. It reads "Showing the 180 days before . Sessions are listed for up to 180 days at a time." Narrower filters show nothing, since nothing is cut. With no date filter the tab lists the last 180 days, which the card footnote already says. It applies to both session fields the tab can query (the sessions API, and the Duo session artifacts field when the ai_governance_sessions_api flag is off).
• The dashboard docs sentence for the card says it lists sessions from the last 180 days.
• One new translation string and one changed, locale file regenerated.
• Jest specs for the footnote and the alert: hidden without a date filter, hidden at 180 days or less, shown with the end date for a wider span, shown when only "Created before" is set, and counting to today when only "Created after" is set.

Screenshots

Real captures at 1440 px, the dashboard in three columns. The card is cropped to its footnote.

	Before	After
Recent agent sessions card
Audit events tab, "Created after" more than 180 days back

Merge order

This change ships in three MRs:

Order	MR	What it does	Depends on
1	Show the 180-day session window on the AI Gover... (!265843 - merged) • Dheeraj Joshi • 19.6	Frontend notes that explain the window	none
2	Bound AI governance session listings to a 180-d... (!265842 - merged) • Dheeraj Joshi • 19.6	Sessions list: 180-day default window and a capped count	!265843 (merged) (merge order, not a GitLab block)
2	Bound Duo session artifact listings to a 180-da... (!265854 - merged) • Dheeraj Joshi • 19.6	Session artifacts list: the same window and cap	!265843 (merged) (merge order, not a GitLab block)

Merge the frontend MR first or together with both backend MRs, on the same day. If only the backend merges, sessions older than 180 days drop out of the lists with nothing saying so.

Related to #635324.