Added API for getBoardPostSummaries

[defnax]

is required to load boards post on webui faster
webui on channels is faster, it using the getContentSummaries

chatgpt says (5.6)

Without the libretroshare addition, WebUI only has:

• getBoardAllContent: returns every post, comment, vote, and image in one large response.
• getBoardContent: returns selected items, but requires message IDs that WebUI does not yet know.

So WebUI cannot request only the newest 25 posts without first receiving their IDs. It can improve the experience with loading indicators, caching, or prefetching, but the initial large download would still take 20–30 seconds.
The small getBoardPostSummaries() core API is necessary to make the first page genuinely load faster.

[defnax]

RetroShare/RSNewWebUI@88aaf7c

Implemented true progressive loading for large Boards
Changes:

• Added getBoardPostSummaries() to libretroshare.
• Returns only lightweight top-level post metadata, excluding comments and votes.
• WebUI sorts summaries newest-first.
• Complete posts download in batches of 25.
• First page renders before older posts finish loading.
• Older RetroShare cores retain the existing getBoardAllContent fallback.

[jolavillette]

please review pr/365, needed by RSNewWebUI pr/121