[FEATURE REQUEST] Support capture default/placeholder value in popup dialog

[ruinshe]

Is your feature request related to a problem? Please describe. Currently, when using a capture template like:

- @ **{{time}}** {{value}}

it is sometimes necessary to change the {{time}} part. The present solution is to run QuickAdd and then manually edit the target file to modify the prefix. I would like an improved workflow where the default value (placeholder) can be written directly into the popup dialog for capture, so that a default message with a specific prefix appears and I can add extra notes before submitting.

This request is different from #461 : I want the default value to be written into the popup, rather than processed via templating or using {{value:xxxx}}.

Describe the solution you'd like Support for user-defined placeholders or default values in the capture popup dialog. For example, allow setting a prefix (like **{{time}}**) that automatically appears in the input dialog, so the user can simply add extra text after it without modifying files afterward.

Ideally, the dialog would pre-fill with a default value, and the user could edit it before confirming. This would make workflows quicker and more flexible for customized capture templates.

Describe alternatives you've considered

• Using templating like {{value:xxxx}} — but that processes static content and does not allow pre-editing in the popup.
• Manually editing the target file after running QuickAdd, which is not efficient.
• Existing solution in #461 which covers writing values outside a set list in the suggester, but not pre-filling the input dialog with a default value.

Additional context Use case: Pop up a capture dialog to add a journal entry or note, where the prefix includes a dynamic value (such as date/time) and a custom starting message, and allow further editing before confirming.

Example desired input in dialog:

@ **2025-12-03** 

And user can write more text after this prefix.